The chords you have been using so far are C, F and G, which correspond with the first, fourth and fifth notes of the C major scale. These three chords are therefore referred to as chords I , IV and V in the key of C. Chord numbers are almost always shown in roman numerals.
This system of numbering chords is particularly useful when transposing music from one key to another, or when explaining a new chord progression to another musician. The following diagram shows how these numbers apply to the 12 Bar Blues progression.