Then you should do a little research first. This is a great article that discusses some of the guidelines Facebook has in place regarding contests or sweepstakes. It’s not that you can’t do them, you just can’t do them willy nilly on your own, without working with Facebook. If you did, Facebook could shut down your page.
If you’d rather skip right to the horse’s mouth, here are Facebook’s Promotional Guidelines.
Nala
Nala Henkel is a social media partner with FEC Network Inc. and editor of the IAAO Blog.

