To become a Tank Commander in the U.S. Army, you have to be an Enlisted soldier (not one of those fancy-pants officers) and have achieved the rank of E6 Staff Sergeant. You'll also need some experience inside of a tank in other positions, such as Loader or Gunner. You'll also need an exemplary track record to prove that you're capable of commanding a team in combat.
For the Marines, the requirements are much the same but the amount of certifications (proof that a soldier is proficient in certain knowledge or talents) is higher and stricter than in the Army. Just don't tell an Army Tank Commander that. Trust us (we still have the bruises).