The Juris Doctor (J.D.) degree from an American Bar Association accredited law school is the credential you need if you wish to practice as an attorney in (almost every state) in the US.
Law school comprises three years. The first year typically includes legal writing and research instruction and courses on the most fundamental aspects of law; civil procedure, torts, contracts, property and criminal law or procedure, and American constitutional law are common first year subjects. The second and third years offer students the opportunity to customize their course work.
In order to practice law, graduates must pass a state Bar exam, which, in most states, includes a state specific portion, a multi-state multiple choice exam, and an exam covering professional ethics. State bar associations have local Bar Associations, but the great majority of law schools are certified with the American Bar Association (A.B.A.). You should take the bar exam in the state (or states) in which you expect to practice, as passing in one state does not necessarily admit you to practice in others.