Learning a 12-bar blues is one of the fundamentals of beginning to play guitar. The basic blues is very simple to learn, and is common ground for guitarists - it can be used as the basis for guitarists to play music together, even if they've never met before. This lesson outlines how to play a 12-bar blues in the key of A.
The Blues Intro and Outro
A blues typically uses some sort of musical introduction ("intro") before launching into the meat of the song. The guitar tab above (learn to read guitar tab) is an example of a very simple intro and outro, which you can memorize and use. This is a very basic blues intro, which leads immediately into the main part of the song. It will take a little practice to play quickly, but this intro shouldn't be too difficult.
The second line of tab above is a basic blues outro that will wrap up the song, the last time you play it. It's not very long, and shouldn't be too tough to learn. This outro begins on the 11th bar of the 12 bar blues, which will make much more sense once we learn the rest of the song.
Once you've mastered the intro/outro above, you should try experimenting to vary these patterns, to make them sound a little more interesting.