They have a capo/fret number matrix that you can print out for reference.
You have to transpose the song. Enter the chord(s) and the original key of the song. The key is usually the first chord in the song.
Pick a new key, (thats where the matrix comes in. When you hit "transpose"
it will display new chords. Use the capo on the appropriate fret and play those chord shapes. IE-if original is in C, and you change it to A, you would put the capo on the 3rd fret and play the new chord shapes. In this example you only change the chord shapes you play. The song should still sound the same. Play along with the original, and there should be no difference in sound.
Sometimes you have to try several different new keys to find chords that you can play, if you are looking the play simpler chords.
If you want to change the key of the song for singing ease, play the new chords without a capo. This raises or lowers the pitch of the song.