Many songs can be played using only 3 chords, and once you have practised a little you should be able to play a song 'by ear' working out the chords as you go along.
The other usefulness of the 3 chords is that when you play a song and it is pitched too high or too low for you to comfortably sing it, you can easily change to a different key just by using the same 3 chords in a different key.
To start off with, these will be your two main keys.
When you see a '-7' chord, you can just play the regular chord (although it will sound 'cooler' later when you use the right one!) So at first if you learn C, F, G and D, that will mean you can play in two different keys.
If you have practised one or two chords, then it is time to play some songs.