Start with the root notes in your piano sing the melody or lyrics of your song that u have written then try to match the key of your song with the composition of your song then use the root notes for the chords of your song start with four root notes for example of you are singing in Aminor Then the root notes will be A,F,G & E(notes only not chords) then lets say it fits in your songs then just star adding the the harmonies ove the root notes and build the chords upon it and then make them in advance add inverses or diminished or add9 of add7 it totally depends on you how you wana portray your song. Keep trying untill it sounds best to “YOU” and then keep progressing.