Choose your equipment. You can play blues guitar on acoustic, electric or a semi-hollow body guitar. Your guitar doesn't need to be fancy and neither does your amplifier. All you need besides a guitar and a starter blues method is the desire to squeeze the emotion of the blues out of your instrument.
Purchase a beginner's blues guitar method book to get you started. You can also find much of this information free of charge online (see Resources). Blues chords are major and minor triads. These are simple three-note chords. Sometimes the flatted seventh note of a scale is added to the chord. A beginner's blues method will show you how to make these chords and how to use the blues notes of a scale (flatted third, fifth and seventh notes of a scale) over these chords. A simple C blues scale, for instance, contains the notes C-D-Eb-F-Gb-A-Bb. These notes will sound good played over a 12-bar blues progression in C.
Jam until your fingers fall off. The best way to learn the blues is to pick a key and record a 12-bar blues progression in that key using the major and minor triads as described in Step 1. A 12-bar blues progression features four bars of the first chord in the key you're playing in--this will be a C if you're playing a C-major blues scale. Follow this with two bars of the fourth chord, two more bars of the first chord, one bar of the fifth chord, one bar of the fourth chord and two final bars of the first chord. The chords you play will be C-F-G. You can play these as seventh chords as well. Your beginning blues book will show you more progressions, but once you've mastered the patterns, it becomes easy to shift from one key to another.
Master the techniques that will help you make your notes cry. Sliding is one of the most useful techniques. The blues don't need to be perfect. You simply want to make your music feel authentic. Sliding from one note to another adds an authentic blues flavor. You can do this with your finger or you can use a metal slide that fits over your finger. Another useful technique, and one you should practice regularly, is string bending. Play a note and push the string upward or pull it down on your fretboard to make your note go sharp. Add a touch of vibrato (gently vibrating your finger to make the note waver) and you're home-free. B.B. King can make a single note say everything with this simple technique.