First, find the scale of the song. For that hum the tune of the song a couple of times and try to find the particular note which brings the whole thing to a pleasing, satisfying conclusion. The tune of the song is built on the scale related to this note.
Next, find whether the scale is a major or a minor. For this strike the chord of the note that you located while humming the tune. For example, if 'C' is your note, first try humming the song to C major chord. If it sounds odd, try C minor. If you have a sharp ear then you can easily see which one fits.
Once you have located the note and the scale, start adding chords as you hum the song. This is not difficult if you know all about chords families (Refer the table given below for help) . Apply the 'Three chord trick'. For example, if the song is in C major scale, then you should be able to play the song reasonably well using 'C', 'F' & 'G7' chords. Remember that chord progression most often depends on some key notes appearing in the tune. So if you are able to play the song on an instrument, locating the related chords would be much easier.
