Familiarize yourself with blues scales. You may want to keep a chart of blues scales handy for reference because there are a number of scales that can be used to play the blues. Primarily, though, a seven note major scale with a lowered third, fifth, and seventh note is used. It's a good idea to familiarize yourself with the major and minor scales as well. You can find blues scales and major and minor scale charts online (see Resources).
Learn chord progression based on the blues scale. Start with a 12-bar blues pattern that uses the 1, 4, and 5 chords of the key you're playing in. This means you will play the chord built on the first note of the blues scale, the chord built on the fourth note of the blues scale, and the chord built on the fifth note of the blues scale. The 12-bar progression looks like this: 1,1, 1, 1, 4, 4, 1, 1, 5, 4, 1, and either the 1 chord or the five chord. These numbers are typically written as Roman numerals. Ending this 12-bar progression on the V chord results in a turnaround. Learning this 12-bar progression will take you far playing the blues on the piano.
Apply your 12-bar blues chord progression and your blues scale to the piano. Use your left hand to do the chord changes and your right hand to play a melody. Start with a simple, slow melody and progression. Use the notes in your blues scale over the 12-bar progression, and you will hear the blues immediately.
Experiment with different tempos using the same scale. Try playing a walking bass line with your left hand rather than chords. A walking bass line is a musical passage built from the blues scale with notes of equal value (typically quarter or eighth notes) played over and over again in succession, creating a "walking" feel.
Practice playing simple beginning blues sheet music. You can purchase beginning blues piano books at your local music store or find them free online (see Resources).