I am not a specialist of harmony, but may be a first advice could help ?

Can you enter the melody of your song in Band in a Box ? You have to choose a tonality first. In C major and A minor there are no flats and sharps (except a G# often found in A minor). Il you choose another tonality, band in a box indicates the flats or sharps at the beginning of the score.
Enter the melody and try to decide what are the most important notes Inside every bar. Normally, these notes belong to the chord played at the same time as an accompaniement.
Try to enter some simple major or minor chords and observe the score of an instrument like the piano or the guitar. Il you can read music, you will see if there are notes of the chord that correspond to notes in the melody.
Normally, sharps of flats in chords are the same as in the melody. Pay attention to major and minor chords so that all the notes of the chord belong to the scale .
The last important topic is the place of the chord in the scale . For instance if the song is in C major, C is the first note(I) and G is the fifth (V). Band in a box can display IIm-V7_I instead of Dm-G7-C. (a very popular chord progression).Most important chords are I,IV and V7 in a major tonality. Then IIm and VIm may be used to vary the accompaniement. In minor tonalities it is a bit more complex (I tell you later) .
Also observe simple songs if you cant get a ready for use Band in a box file of them .