The 12 bar blues is easily one of the most popular chord progressions in music. If someone asks you to play a “blues in G”, they are most likely referring to a 12 bar blues. There are other variations, such as the 8 bar and 16 bar, but 12 is the most prominent. This progression is usually broken up into three groups of four measures. Typically, the singer will sing for the first two measures followed by two measures of guitar (or some other instrument). This is called call and response. The singer calls, the guitar responds. The last two measures of the progression are the turnaround, which signals to everyone that you will be returning to the beginning of the chord progression. Normally, you will keep repeating the progression until the song is over. Even though the progression repeats, there are solos, stop time sections, and many other devices used to keep it interesting.
The standard blues progression is based on three chords, the 1, 4, & 5. Usually they will be written as roman numerals – I, IV, V. How do you figure out what these three chords are? First, the one chord is the key of the tune, so if you are playing a blues in C, the I is C. To find the IV chord, you count up the scale, C=1 D=2 E=3 and F=4. You would continue on to find the V chord, G. So if you are in C, the I is C, the IV is F, and the V is G.
Below are a few popular keys:
A, D, E
E, A, B
F, Bb, C
Bb, Eb, F
Obviously there are more keys, but these five (including C) are enough to get you started.
If you are not that comfortable with theory, I have a diagram to help you. 
These show the relationships between the root of the 3 chords. If the I is on the second fret of the 6th (low e) string, then IV is next to it as the second fret of the 5th (A) string. The V is then two frets above the IV, or the fourth fret of the 5th (A) string. This works the same way as scales and bar chords, if you change the key of the I chord by moving it up three or four frets, both IV and V move up with it.
This works the exact same way if the I is on the 5th (A) string. However, instead of IV being next to it, it is two frets lower on the 6th (low e) string. V is next to the I chord on the 6th (low e) string. You will probably not use this as often as playing the I on the sixth string, but it is good to know.
One thing that is important to remember is you will not always be playing chords if you are the accompanist. Sometimes you will be playing the shuffle pattern or a riff like what is used in Rock Me Baby. If chords are called for, is a safe bet that you should use seventh chords.
Below are two 7 chord shapes, one with the root on the 6th and another with the root on the 5th that will fit nicely into the I, IV, V pattern we just discussed. You will only play 3 notes at a time, and if there is no finger on the string, don’t play it! With these two chords, you can play in any key. The 1 means use your index finger, 2 is middle, and 3 is ring.
Now that all of the background stuff is taken care of, you can play the progression.
The 12 bar blues.
Here’s a YouTube clip demonstrating this in Bb, with the root on the 6th fret of the 6th string. Each chord is played at the beginning of each measure, to reinforce which measures contain which harmony.
The following progression has a few examples of variations. If you want to do a quick IV, you will play IV in the second measure, going back to I in the first measure. There is also a V chord in the 12th bar. Both of these are popular, and you will need to listen to the tune you’re playing to decide if you should add them or not. Usually, if a tune is going to use the quick IV, it will do it every single time. You don’t just add it or take it out in the middle of a song.
Here’s a YouTube clip demonstrating this in E, with the root on the 7th fret of the 5th string.
Have fun!
The Blues 