Ok, I'm a big fan of Watchmen, and I'd highly suggest you to read Watchmen before going to see the movie.

Here's the link to the official site of the movie:
http://rss.warnerbros.com/watchmen/

And also, here's to buy the book: http://www.amazon.com/Watchmen-Alan-Moore/dp/0930289234

If you like it, buy it!