You have to be at least 15 and head up to the dmv with you birth certificate and certificate of attendance. You must pass a test on road laws and traffic signs.
I would suggest taking practice tests on-line before taking the actual test since you have to pay to take it.
This varies wildly depending on your state of residence. Try the state DMV website. Almost all states will require several forms of ID. Be prepared with, at a minimum, your birth certificate. You will almost certainly have to pass a test to get the permit, so be prepared to put in some study time. Most states I have lived in have the booklets you will need to study available in DMV offices and online.