12 bar blues is basically a I-IV-V chord progression.
For example, in the key of Bb:
Bb/Eb/Bb/Bb/
Eb/Eb/Bb/Bb/
F7/Eb/Bb/Bb/
(as you can see, there are 12 measures. I=Bb, IV=Eb, V=F7)
Here's another one, in the key of G:
G/G/G/G7/
C7/C7/G7/G7/
D7/C7/G/G
(I=G, IV=C7, V=D7)
There are different forms to play. The above example isn't the only way. As for the chords themselves, there are also different ways of playing each measure. The 6th of a chord is usually involved. Ex: Bb in the first measure can go: Bb D F G.
There's lots about 12 bar blues on this forum. Try doing a search, I'm sure you'll find much more info.