It's different in every state, I think, but in California you're eligible for a Permit when you're 15 and a half. The permit allows you to drive with any licensed driver 25 years or older in the front seat with you, and to get one you have to complete drivers ed (theory), which I did online. The drivers ed is really a big joke, if you go to the classes you actually learn, but the website I did it on was a complete money scam. You just have to pass 5 ten question multiple choice quizzes, with a score of 7 or higher, and you have as many chances as you like, so you just guess and most likely pass them all first time.
Once youve finished that, and enrolled in a behind-the-wheel school, you can apply for your permit, which you get by passing a 50 question theory test, which is really easy. I just guessed them all and passed
After you've held a permit for 6 months, are 16 years old, and have finished 6 hours of professional driver training, you can take the behind the wheel test, which i failed.