Qualifications
You may not have to go to college to be a roofer, but that doesn't mean you can just roll out of bed one morning, throw on some clothes, and start roofing. Sorry to be a buzzkill. There are a couple of hoops you'll need to jump through, but they're really for your own benefit.
Unless you plan on trying to roof huts in a rain forest somewhere, you're going to need a contractor's license and possibly a business license in order to be an official roofer. Not only do those licenses give prospective clients a reason to trust you, they also give you some legal standing in case a client fails to pay. Working for nothing gets old pretty fast, so if you're going to go toe-to-toe with Stingy McScrooge, you'll want the law on your side.
Specific licensing requirements vary from state to state, and just because you're licensed in one state doesn't mean you can just move to another and work off that same license. General requirements are having a high school education, being insured and bonded, having proof of citizenship, and meeting the qualifications to submit bids in your state (source).
Many states require contractors to carry separate insurance to cover accidents on the job, such as someone getting hurt while working for you or doing damage to your client's property. Just for example, let's say someone on your crew drops a couple of roofing nails in your client's driveway and the client runs over them and shreds a tire. You'll be responsible for replacing the tire, pronto. And you'll be glad you're insured.
Once you've gotten all the paperwork requirements done, you'll just need to be friendly, professional, hardworking, and realistic about what you're promising. Clients will appreciate you taking the time to do the job right. You can, and will, take that to the bank.