Determine what key the song is in. The easiest way to do this is to look at the last note of the song. Songs almost always end on the tonic, or root note of the key. If the song ends on an A and feels like a pretty happy song, then it's a good bet the song is in A major. If it sounds a little sad, it's very likely to be A minor.
Pop songs have relatively easy chord structures and usually follow a predictable pattern of using I, IV and V. Figure out what the chord structure your song is. For example, in C major the chords are likely to be C major (I), F major (IV) and G major (V).
Once you know the chord sequence, for instance I, IV, I, V, I, you can decide if you want to transpose your song higher or lower. It's easiest if you only move one step up or down at first until you have the hang of transposing your song.
Transpose your song down 1 step by moving backward in the alphabet on letter. C becomes B, F becomes E and G becomes F. Try playing your song with the new chord sequence. Still too high? Go another step down using the same process.
Transposing your song up 1 step uses the same procedure, only in the opposite direction. C becomes D, F becomes G and G becomes A.