I want to do a contest and I don't know anything about it.

You can find information about doing a Facebook contest by clicking the links below:
http://www.votigo.com/corp/self-serve/faq.php
http://support.votigo.com/entries/20560237
http://support.votigo.com/entries/21634963

You can also find Facebook’s promotional guidelines {here}

Click here to Register and start creating the contest of your choice.