1. Take a look at this chart, it represents the circle of 5ths (open in new window). The outside letters represent the major keys and the inside letters represent their relative minors. If you don't understand what that means, thats okay for now, keep reading.
2. Okay, pick a letter on the outside of the circle, any letter. That will be your (major) key.
3. Now draw a circle around that letter that also captures the letter to the left and the letter of the right. So for C your circle would include the F and G. Your circle also includes the minors on the inside.
4. Now look at what you have. For C, for example, we have C major, F major, G major, also A minor, D minor, and E minor. And guess what, thats your answer. Grab a guitar and try it out. Mix them up. They will all sound "right" together.
5. Finished. You can make this circle around any letter and now you know the 6/7 chords to make a song in that key. The missing chord is the diminished chord that is rarely used anyways (but look into it if you want to).
6. Also, to play chord progressions in the "Minor Key", just treat the minor chord as your home. Example: end or begin on the A minor instead of C major, you can use the same 6 chords.
So yeah, grab a pen and paper and write down the Circle of 5th's as I have it linked. Put it into your guitar case or music folder. Now you have the answer at your fingertips.
Boom. Songs in every key.