Well, you may want to check out Elance which will allow people to find you. I would also recommend having your own site which also links to a portfolio of demo sites. From there its a lot about just getting in contact with people, or knowing people. I, for example, get most of my freelance jobs from friends of friends. Most of the people I know and work with know I do freelance web work on the side, so if they have someone who needs a website, they send them my direction.
If you are looking of starting a more formal 'business' or something like that, I think that will be a whole different ball game. I would recommend going the routes above, get some contacts with people, and make connections. Knowing people will, in my experience, get you more work than most anything else.
I was in a similar situation you seem to be facing. A few years ago I was someone who was looking to get freelance jobs, and had a number of years worth of experience. I will tell you that, especially if you choose the more business like approach, you age will be a factor against you, at least at the beginning. Being only 21 myself, I know that it can be tough to get the job when going up against someone with 10+ years of experience, but that is where an impressive portfolio will be most helpful. You need to make sure people know what you can do, otherwise you'll probably struggle.
And finally, I wouldn't recommend trying to do freelance as a full-time supporting gig to start off with. It can be great to make money on the side, but it can be tough, even for those with lots of experience, to keep enough jobs to support yourself. I would recommend checking out local job boards, including those here at the stackoverflow family, to try and find jobs in your area for web designers full time.
Hi. My biggest problem working in this field was the sort of clients I had to endure. The first guy I worked with pretty much took advantage of me and expected me to do tons of free work. Other wise known as Spec work. Lots of people who come to you will try and get you to lower the price because they do not want to spend too much on a site. Some people are a nightmare to work with. Those online sites for advertising are generally people sending work of to some 3rd world country like India for $30 instead of paying us in the states. I use to use those sites but it was horrid how it was near all out-sourced. Your age does go against you. Though at the same time, you have a great skill set at such a young age. When you do work freelance, make sure you have in-depth contracts drawn up. Put on the contract exactly what you are doing and exactly what you are charging for. People do and will try to take advantage. Stand your ground with your prices as you gotta pay bills just like they do. Do not do any spec work for " local employers " I made that mistake. Your portfolio / resume is all you need to show potential employers and clients. Any work you do,, get a 50% deposit up front to cover costs and your time. Advertise locally in the newspaper, go round a few local stores, just try hard to get your name out. Look at what the other developers in your town are charging, how good is their work?.