To find the 'key' of a song, press a note on a keyboard while the track is playing, change the note until it 'sounds right' for the most part of the track. One should 'sit' in place. That's the key you're in.
Harmony - even if you know nothing about musical theory, for each individual note in your main melody, isolate it temporarily (loop or something), then play different notes on the keyboard until you hear a note that isn't the same as the melody root note, but that sounds good, and compliments it? Simple?!
As someone else said, start with notes, rather than chords.