Whether your addiction is to social networking, surfing the web, instant messaging, e-mail, or gaming, I have a solution that might work for you. Have your parents buy you a netbook. They are mini laptops made for extremely simple tasks, and they are only a few hundred bucks. You can really only type papers and go online with one because they simply aren't made for anything else. Unless you are addicted to Solitaire (I had a serious addiction to it) or those included games which you can uninstall anyways, you won't be gaming. As for the internet, if your family uses dial-up or cabled internet, you will have no internet. :-) Problem solved. If you have wi-fi, have your parents encrypt and refuse to let them tell you what the code is for your wi-fi to work. Then, ALL you can do is type on there. No optical drive for discs, distracting media, internet, or games.
If you cannot convince your parents to get you one even with the above reasons, consider getting help from as psychologist or school counselor. And if your school lets you, stay after school in the library, a classroom, or computer lab and work there. You'll keep on track and it will be unlikely that you'll start playing games on their computers if there are any. Just type your papers there.
Another idea is to award yourself computer time for doing homework. For example, for every hour you do your homework, you get 15 minutes of computer time. At the end of the night AFTER the homework is all done, use up your time and feel good knowing you earned it. As you get less addicted if that ever happens, reduce the number of minutes to a reasonable amount. The 15 minute rule isn't bad. Work for 4 hours, play on the computer 1.
It's nice to see that you have realized you have a problem and need to remedy it. That's the first step. :-)