Guitar chord diagrams represent the position of the fingers of your left hand, and how they press down on the strings to make the chord sound when you strum.
The guitar chord diagram is a visual representation of the guitar's neck.
The numbers on the finger positions (the black round circles), correspond to the fingers in the diagram below:
A small "x" on the top of a string means that you do not play this string.
These photos show how the fingering from tha chart translates to the real world.
The number 3 on the chord diagram, corresponds to the third fret along the guitar's neck.
Start by learning the chords in the key of C.
"What chords are in the key of C?" you say - remember to look at your circle of fifths in the music theory section of this site.
Once you have mastered the chords in C, move on to G, then D and then choose whatever you like. Have fun!