Good voltage on the Battery does not mean the battery is good. when they fail they still show proper voltage. but when you try to crank there is not enough AMPs to get it to crank. getting another car to jump it does not help either.

how old is the battery? if it's more than 5 years that's probably your problem.

Starter failures are very very rare...what engine do you have?

most likely is your battery.. if not possibly your harmonic balancer.

even if you buy a new battery if it's not properly charged it will not start the car.