Circle of fifths. Picture a clock. At 12 put the note C go round the clock putting the note that is a perfect fifth in the next spot. So; C, G, D, A, E, B, F#. Now of course the problem becomes do we call the next note C# or Db? And I can't remember where the shift hits the clock. Its a memory aid (when you can remember it) for some useful progressions. With the circle of fifths firmly locked in your consciousness you can look at it and know that the three basic chords for the key of G are: G and the ones on either side of it, C and D. There's more about it that's useful for people who know more about music than I do.