He did not attend Law school, serve as an apprentice, or take a Bar Exam.
Abraham Lincoln became a lawyer under an Illinois law enacted in 1833. This law stated that to be a lawyer someone had to "obtain a certificate procured from the court of an Illinois county certifying to the applicant's good moral character."
On September 9, 1836, a license to practice law was issued to Abraham Lincoln by two of the justices of the Illinois Supreme Court. Later, in a more formal session, on March 1, 1837, Lincoln appeared before the clerk of the Illinois Supreme Court and took an oath to support the Constitution of the United States and of Illinois. Lincoln was then formally enrolled as an attorney licensed to practice law in all the courts of the state of Illinois.
Lincoln prepared for his career by reading Blackstone's Commentaries, borrowed from Stephan Logan, an Attorney, who would later become Lincoln's law partner. "Abraham Lincoln's early legal training and the rapid changes in antebellum law ensured that his legal education continued throughout his law career. Lincoln's early career as a lawyer was a vocation, and an apprenticeship, rather than a formal or informal learned study of the various subjects in law, followed by a formal standardized examination.
Although Lincoln advised would-be lawyers to 'still keep reading' after becoming licensed, Lincoln's reading instead was directed toward the case before him.