An Individuals faith in the principles of religion, endeavors to find accurate instructions from the text of the Quran and also from the statements and practices of the Prophet (Book & Sunnah) and acts accordingly in good faith.
Anyone who understands Quran in depth, finds true Hadith in his searches and understands his own obligations which religion imposes on him and at the same time realises the collective or individual obligations as religion has imposed on him and then makes a judgement, he becomes a Mujtahid. Conditions of Ijtihad.
From the word Ijtihad and Mujtahid we must understand that Mujtahid is the person who endeavours to understand deep meanings of the Quran and Hadith with the sole purpose of finding out the exact instruction from Allah and gives a decision on that sound basis. With the sound knowledge and good memory , he should also have the following qualities in order to be eligible for the title.
1. Knowledge of the Arabic Language to the extent that he becomes well versed in Arabic literature, its grammar and its eloquence.
2. Identifying the narrators of Hadith that he is satisfied about their qualities as narrators. Technically this is "Ilmeu Rijal" (Science of Men).
3. Understanding various stages of Hadith and identifying correct Hadith from non correct . This is "Ilmu Rewayat" (Science of Narration).
4. Acquaintance with the Rules and the Principles of Fiqh to the extent that he is able to apply each branch of observances to its base with full authority. This is 'Ilm Usool-eFiqh' (Principles of Jurisprudence).
5. Having full information on the exegesis (Tafseer) and meaning of the Quran and ability to absorb and apply relevant verses of the Quran. This is 'Ilm Tafseer', Exegesis of the Quran.
6. Having sufficient information on general knowledge, specially mathematics, history, geography, astronomy, and the social studies which are required in his dealings in day to day matters. He should also be acquainted with things like knowing the direction of the Qibla, divding the inheritance and some economic and medical knowledge so that when people ask him about these he should not remain silent.
Apart from all these he should be understanding and accommodative and knows the ways of argument. This way the Mujtahid should know his own obligations and be able to convey the same to others.

Marja'ia:
The person of religious authority whom people refer to in religious matters. If a Mujtahid possesses all the good qualities mentioned above and an ordinary person accepts his instructions on observance of religious duties in addition to fulfilling the following conditions:
1. Adalat: Means being unsinful to his best ability
2. A'alam: Means his circle of followers is higher than others and guides all Muslims in the possible manner. He also puts them in a satisfactory position in all religious matters.
When such person due to his qualities of Adalat and knowledge becomes a focal point for persons to follow, and prominent religious persons who endorse that view, then that Mujtahid becomes "Marja-e-Taqleed", and to follow him in religious matters is called 'Taqleed'.

Taqleed:
If a Muslim who himself is not a Mujtahid because his vocation was different and he was not able to acquire sufficient knowledge, then he follows a Mujtahid upon whom he has full confidence. In other words, on all religious matter he follows the judgement of that Mujtahid. In this kind of following where this person has asked the Mujtahid and followed his instructions, would in turn makes that Mujtahid responsible if any mistakes had occurred. He has transferred his religious obligations on the shoulders of the Mujtahid.
This is only reasonable that in every day activity and in every profession there are specialists, and people approach them to satisfy their needs. For the purpose of an operation one would not approach an architect or a businessman but a doctor who has specialized in such operations. Similarly in matters of religion one approaches an expert opinion and a correct treatment for his spiritual and religious ailment. In everyday life we see a child learning by copying from his parents, their speech. their manners and so forth. A person cannot be an expert in all professions and he asks experts when he needs advise. When this advice is given by an expert he follows on that advice. This is 'Taqleed'.
But there is one kind of Taqleed which is not positive and therefore undesirable. This is blind following. Blind following does not conform with the Islamic spirit. People in the world do try to follow blindly, like following the fashion trends in dress and decorations. This is against intelligence. With this blind following the danger is that undesirable activities and traditions become part and parcel of life and in time they become an irreplaceable part of a society. They were followed without giving rational thought, but after a while when the same unreasonable things become part of life it would become impossible not to follow them. Man would then become a prisoner of these traditions and he would lose the ability to follow reason and intelligence. There are societies that were formed in the same way to the extent that if rational persons tried to detach themselves it would become impossible. It is for this reason that blind following has been regarded as something endesirable.

Distinctive Qualities of a Marja'
A Mujtahid who has become a Marja' because a large number of Muslims follow him and he solves their religious problems in accordance with the Book of Allah and Hadith, should also have the following four distinctive qualities:
1. Seyanat e Nafs: This is self protection from sins. He should be able to protect himself from activities which are regarded as mistakes or sins of minor nature.
2. Hifz e Deen: This means he should not have ant attraction for money or position and his whole life should be surrounded by 'Deen' and all his activities should be under the light of Islamic Ideology and his should always be protection and preservence of Islam. His sole purpose in life should be obedience to Allah (SWT), nothing less. In other words he should never have any ideas on obedience to anyone but Allah. He should not be a tool in the hands of anyone in the world.
Imam Ja'afar Sadiqe(as) in connection with the Ulemas who have their own following on religious matters and they take all responsibilities on their shoulders, has given a clear directive on this matter:
"From among the Jurists (Fuqaha') who protect themselves from sin, safeguard Deen, act against their own desires and are solely obedient to Allah are those whom Muslims should follow in Taqleed" Similar directives has been received from our 12th Imam on the distinctive qualities of the Fuqaha' who become centre of religious authority (Marja'). It is possible that each day a new problem emerges which has no parallel before or was not discussed previously, Muslims require the assistance of a Mujtahid, in every age and time who should be able to fulfill their religious needs and solve their problems in a proper manner.

Duty or Obligation ?
Every individual Muslim, in every aspect of his life, is under obligation and duty bound, as far as religious problems are concerned, to identify the problem, find the solution and act upon it. This duty or obligation imposed on every Muslim on identifying religious problems and finding its solution is called (Takleef Shari'I).
Limitation of his Obligation.
There is an age limit when this duty becomes obligatory. 1. This is a specialist's assignment for all other Muslims. If one individual Muslim has not been able to acquire such knowledge and qualities then he has to consult the Marj'a.
2. Like the question of Life Insurance, Banking, permissible or non permissible, opening of the Caracas at the time of Sacrifice at Mina, economic and other problems of the society or even announcement of the sighting of the new moon for Eid Al-fitr in order have a united Eid.etc.
3. For a male person, the age limit is about 15 years, less or more.(Signs are attaining puberty and hair growing on private parts). For girls it is the beginning of menstruation or hair growing and it is commonly 9 years of age. When these signs appear, the male or female person should realise his or her obligations and duties in order to observe or perform religious duties. Allah has given each individual strength and powers according to his abilities. Therefore , duties are also obligatory on such.

Religious Commandments as Obligatory
These are Ahkam Takleef, But there are Ahkam Waze'e:
1. Ahkam Takleef are those studies in which the person has the chance and the intention of his own whether to perform them or not.
2. Ahkam Waze'e are those in which a person has no chance to prevent, or had no intention of doing them but they happened inadvertently. For instance during sleep one stretches his legs and damages someone's valuable property without intending to damage it. In other words any act which was wrong in its substance but occurred unintentionally. This is called in Fiqh 'Ahkam Wazi'i.

Types of "Ahkam Takleef "
All human actions have a definite order and they are classified in the following five divisions:
1. Wajib (Obligatory): The Commandments that must be performed by an adult and their non performance is considered a sin for instance. Daily Salat on an adult its neglect is sin. Identification of Religious Problems and participation in the problems of Muslims generally and helping them in their needs.
To spend life searching for a dignified and correct religious stature. To spend life and time for Muslims and to help them in their need even when he himself does not require any help or assistance.
2. Mustahab(Recommended): This is the type of observance if performed becomes an act of goodness and desirable for all concerned. For example; to welcome guests in your home, to remain in a state of Wudhu all the time, to offer Salat in Jamaa' (Congregation), to use fragrance on your body to dispel bodily smells, to brush your teeth with Miswak, to attend to a sick Muslim or brotheretc. All this is Mustahab and there is thawab reward in it.
3. Haram: Things that must not be committed at all, for they are totally forbidden. To act on these forbidden items is equal to sin and its rejection is Wajib ( A Must). Examples are; telling a lie, hurting someone, hoarding goods when other Muslims are in need, oppressing others or helping the oppressors, even making friends with the oppressors, bribe taking, gambling, stealing etc.
4. Makruh (Disliked): Items whose renunciation is better, Like urination while standing, reciting Salat at a filthy place, actions that can be harmful to others like hoarding goods to be sold when prices would go upetc.
5. Mubah (Allowed): Items that have equal significance whether performed or not performed, like eating halal food, going for sightseeing, enjoying a stroll in the parketc.

Conditions of Obligatory Duties:
There are conditions when and whom observance of religious commandments become obligatory:
Apart from reaching the age of maturity, generally beginning of the 16th year for men and beginning of the 10th year for women, it is essential that they should have normal intelligence and understanding of life's obligations. An insane person , a child under age, an unconscious person or anyone who has no control on his body or bodily functions is exempted of these duties and obligations.
In some cases reaching the age of maturity alone cannot be a sufficient motive for compulsion of these observances. Similarly there can be factors that may prevent a person from taking or accepting full responsibility. If a person is generally able to make decisions, like marriage, financial matters, or others signs of reaching the age of maturity, bodily maturity, mental maturity, ability to extinguish between good and bad , and knowing what can be better for his future life, which people he can trust and depend on in life. Then this kind of maturity can be important factors in reaching the age of maturity.
But if a person is simple or just silly, not able to distinguish between good and bad and can easily be deceived by other people, then the responsibility of accepting all religious obligation can be relapsed from him.

Wajib - AINY and Wajib KIFAEE
Obligations that are a must and Obligations that can be taken over by other Muslims. There are some observances that are a must on every Muslim and their observances must fall on his shoulders. He cannot delegate them to others. For example, Daily Salat "Prayers" must be performed by the person himself. No one else can do these duties for him. This type of observances are called Wajib Ainy.
But there are other types of compulsory observances that can b delegated to others, or if others are available then the responsibility lapses automatically. For example the participation in Prayers for a dead person is Wajib (Compulsory), but it is Wajib Kifaee. That is if at least five Muslims are available for the dead person prayers then that obligation has been removed from the shoulders of other Muslims. But there is a subtle point here. One must make sure that others are available. If for instance no one was there and because of that the Prayers lapsed, then every Muslim will be committing sin. So this is also a responsibility that one should find out that others are available.

Source: Noor Al Islam