also, how do i harmonize? i can't play chords in my left and the melody in my right and... have it sound good. just gotta keep playing, i suppose.
the next time someone sings the happy birthday song, try doing this: along with the final 2 melody notes in the song, you can sing above it. for example, if they are singing in the key of F, the last two notes are G F, so try singing Bb and A along with them. magic. To explore more possibilities and learn the mechanics, maybe the best way is to use a piano/keyboard and start messing around and see what you like