Wow. That's a tall order. Let me begin by saying that the very basic chords in any given key are constructed using the scale step as the lowest note which is then called the root of the chord. On top of that is the third 3 notes higher counting the root, and the fifth, 5 notes higher counting the root. On the musical staff they will either go space space space, or line line line. These are the simplest 3 note chords which are also called triads.
The most commonly used chords in pop music are the chords based on the first note of the scale designated by the Roman numeral I called "the tonic", the chord built on the fourth note of the scale Roman numeral IV, and the chord built on the fifth note of the scale Roman numeral V. Very often the V chord will have one more note stacked on top which is the 7th making it a V7 or dominant 7th.
These 3 chords are the major chords in each major key. The chords built on the 2nd, 3rd, and 6th steps of the scale, ii, iii, vi are minor chords. The scale built on the 7th step of the scale vii is a diminished chord.
Suppose the song you wrote is in the key of C with no sharps or flats. Your choices to "harmonize" or add harmony to each measure in the key of C would be to type one of the chord symbols below in the measure and see if you like the sound.
C, Dm, Em, F, G (or G7) Am, Bdim, C
To give C or F a more jazzy sound, you could try Cmaj7 or Fmaj7 OR C6 or F6
To give Dm, Em, or Am a more jazzy sound, you could try Dm7, Em7, or Am7
Some tips: Most tunes end on the tonic chord. Many but not all start on the tonic chord. V or V7 generally leads back to I or maybe vi. IV often leads to V, but it can also go back to I. I vi IV V I is a familiar chord progression from the '60's. Good luck with your song.