Harmony in songwriting does not have to be complex. Some of the best songs have very few chords.
3 Chords G C D (I IV V)
4 Chords I V vi IV
More 4 Chords
Simple chord progression (a place to begin)
F G Am
Get these chords under your fingers and improvise a melody above.
You may want to add a bass line and some rhythm.
Here's some inspiration:
30 Best Rock Riffs