Qualifications
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 (source).
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.