Well, if your just starting out, it is almost an unwritten rule that every guitarist learns the opening riff to Deep Purple's Smoke on the Water. All power chords -- lots of fun (Mind you, very few actually go on to learn the rest of the song!) America's "Horse With No Name" is a real easy song to start out with that will help you get the feel of playing rhythm. If my memory serves me correctly, the song only has about two chords, one of them is Em and the other is this D-based chord:

E___0______________________________
B___0______________________________
G___2______________________________
D___0______________________________
A___0______________________________
E___2______________________________

That may not be it exactly, but that will get you in the ball park. Also, the band, Green Day, uses chords that you know how to play in almost all of their songs. Go to almost any online guitar tab site for their music.