For rock, blues, etc. I would suggest knowing Major, 7th (not major 7), minor, minor 7 and 9th chord forms. That will get you through a lot of popular music.
Also its critical to know the names of the individual notes on the guitar.
Here's a site that might be helpful
http://www.looknohands.com/chordhous.../index_db.html but you can also just search on google for 'guitar chords' and you'll find plenty of sites.
The beginning is the hardest part - just keep playing. It gets to be a lot of fun.