Suppose a song is in C Major, its chord progression will most probably contain the chords from the family of C Major.
Chord Families
Once you get famliar with chord families, finding the right chords of a song becomes easy. You just have to know how to determine the key of a song. Once that is fixed, all you have to do is start adding chords from a chord family. Suppose a song is in C Major, its chord progression will most probably contain the chords from the family of C Major.
C Major/A Minor

D Major/B Minor

E Major/C# Minor

F Major/D Minor

G Major/E Minor

A Major/F# Minor

B Major/Ab Minor

Download FREE GUITAR CHORDS DICTIONARY
Confused about a chord? Try the chord families page, or the guitar chords dictionary.
How to transpose the chords? Major scales can be transposed by simply selecting the desired note given above the song. To transpose a minor scale select the note of the relative major scale.