Learn the primary chords in at least the key of C Major. A chord is made up of at least three notes. The ones you'll need to know are I, IV and V7. Notice how the notes shift. This is particularly noticeable on the keyboard.
Follow the pattern of a blues song. A blues song has measures in I chord, two measures in IV chord, two more measures in I chord, one measure in V7 chord, one measure in IV chord and, finally, two measures in I chord. This is the simplest form; if you want to add verses, just simply start the pattern over.
Add lyrics, if you'd like, that are honest. Usually blues music has a realistic tone to it that showcases life exactly as it is. This is not rose-colored-glasses, life-is-beautiful music. This is "I'm down, and I'm telling you about it" music.