Once you have the melody notes figured out here is a chart I've found helpful to come up with chords that will harmonize with the melody.
1 degree of the scale try I, IV, vi or ii7 chords of that key.
2 degree of the scale try V, ii7, iii7 chords of that key.
3 degree of the scale try I, vi, iii chords of that key.
4 degree of the scale try IV, ii, v7 chords of that key.
5 degree of the scale try V, I, iii chords of that key.
6 degree of the scale try IV, ii, vi chords of that key.
7 degree of the scale try V7, iii, Imaj7 chords of that key.
Key of D looking for a chord to go with the F# melody note. F# is the 3rd degree of the D scale so the I (D,F#,A), vi (B,D,F#), or iii (F#,A,C#) chords will have a F# note in their makeup thus harmonize.
It's a chicken or egg thing. Works both ways -- looking for chords to harmonize notes or looking for notes to use over chords. The chart will give you several choices and your ear can decide which is best.
Have fun.
Malcolm
