A Mujtahid is someone qualified to exercise ijtihad, which literally means striving and technically means juridical endeavor and competence to infer expert legal rulings from foundational proofs within or without a particular school of law.
Requirements of a Mujtahid:
Firstly the Mujtahid must be a Muslim and a person of sound mind and intellectual competence.
*Knowledge of Arabic such than he can understand the Qur'an and Hadith correctly.
*Knowledge of the Qur'an which includes Makki/Madani; Occasions of Revelation; Incidences of Abrogation; LEGAL TEXTS (aayaatul ahkaam). (In short all the requisites of Tafsir)
*Knowledge of the Sunnah specifically the legal texts (ahaadeethal ahkaam). He most know where to find the Hadiths and be able to distinguish the reliable narrations from the weak.
*Knowledge of the substance of the Furu` works and the points on which there is Ijma`.
*Knowledge of Qiyas (Analogical Deduction)
*Knowledge of the Maqasid (objectives) of the Shari`ah
*Knowledge of the General Maxims of Fiqh. eg. Certainty prevails over Doubt.
Another description can be found in Shaykh `Ali Hasabullah's 'Usul al-Tashri' al-Islami' (5th ed. 1976) p. 94-95:
The Mujtahid is he who possesses, together with complete soundness of mind and of Religion, three necessary traits:
1. Knowledge of the Arabic language and the ways in which its signifies meanings. This knowledge does not come except to one who has frequented its various disciplines and read much of the works of its masters of eloquence until he knows how to differentiate between the specific and the general, the literal and the figurative, the explicit and the ambiguous, and other aspects upon knowing which depends his ability to infer rulings.
**He does not have to reach the mastery of language of a Khalil or a Sibawayh or al-Asma`i and others of the Imams of Arabic. It suffices that he attains the rank necessary to understand texts in the right way.
2. Knowledge of Qur'an and Sunna and whatever is in them of rulings, those that were abrogated and those that were not, together with the linking up of the universal with its particulars, the absolute with its restricted sense, and the general with the specific. He does not, in this, have to have memorized all that is related.
It suffices that he has to be able to gather up all that is connected with the topic he is investigating and to know what the experts of hadith have said concerning sound or weak gradings as well as what they said concerning the narrators with regard to discreditation and commendation.
3. Knowledge of the objectives of the Law and of the living contexts of people as well as the customs they share and whatever harms or benefits them, and the ability to know the minute defects of legal rulings and to compare and contrast their similarities so as to better understand facts and infer the rulings that most precisely correspond to the objectives of the Lawgiver and implement the welfare of those under consideration.
Taken from: "PRINCIPLES OF ISLAMIC JURISPRUDENCE" by M Kamali p. 374-377,