You will see that you CAN NOT ask people to buy your product for a chance to win something!
There are several other rules to be aware of if you insist on doing it yourself. Be aware that there are services that are free, such as http://www.rafflecopter.com/ and it would be much safer to use them instead of risking high fines.
This is another link that should help you: http://www.ehow.com/how_4773307_run-contest-website.html
Just be smart and do your research before you get into trouble, it's not that hard.