Look around your neighborhood and town. Look at the web sites for local businesses. Even in 2012, the vast majority of them are crap. Offer to make them for free or at very low cost. Or make one for local churches and/or local charities/organizations, or even bands. That will get you some experience and example sites you can show to prospective customers. And of course, make your own business site (with your own domain, and not on a free hosting service). From there, start advertising. Use craigslist, facebook, flyers around town, etc. There seems to be a serious lack of competent web site designer/developer talent that is accessable to small businesses and groups, particularly in small towns. If you are good, I think you will do well.
You have a huge advantage right now. You're young. You dont have a mortgage or family to support (I hope). You can get away with not charging high hourly rates. That might encourage companies that might not otherwise spend the money on professional designers to give you a shot.
I would make sure you understand, though, that its not likely that you'll be contracted to create high-end line of business apps for fortune 500 companies. You will be starting small. I'd expect you'd be primarily installing and tweaking WordPress and other CMS's. But by the time you are 21-22, you very well may be working on those kinds of applications.