Once you've got the G C and D chords down, learn Em next. Those four chords will let you play literally hundreds of songs....
Usually G, Em, C then D. Off the top of my head:
Stand By Me - Ben E King/Lennon
Twisting The Night Away, Chain Gang, Wonderful World and Cupid - All by Sam Cooke.
His Latest Flame - Elvis (one of many!)
Love Hurts - Everly Bros.
Every Day - Buddy Holly.
Metal Guru and Cosmic Dancer - T. Rex.
Massachusetts - Bee Gees.
Note - not all of those are in the same key. The Sam Cooke songs, for example, are (I think) in Bb or B - so you'd either have to play Bb, Gm, Eb and F or B, G#m, E and F# to play along with the radio or the CD. Or put a capo on the third (for Bb) or fourth (for B) fret and play G, Em C and D.......
"Sometimes the beauty of music can help us all find strength to deal with all the curves life can throw us." (D. Hodge.)