The thing that you need to know is the fretboard, or the notes that make up the guitar. The notes on a guitar go up chromatically. In other words, they go up a half a step at a time. So if you were to start on A, The notes would ascend like this... A, A#/Bb, B, C, C#/Db, D, D#/Eb, E, F, F#/Gb, G, G#/Ab, A. Notice that, there is no sharp or flat between B and C, or E and F. Now study the fretboard and learn the notes. You might not use them in the immediate future. But this should give you a good hint on how to learn the fretboard.