Find chords in the major scales as the I-IV-V: that means the Tonic(I), the sub-dominant(IV), and the Dominant(V) based on circle of notes of the key of the song. For the minor scales the roman numerals for chords are lowercase i-iv-v: that's the tonic(i) sub-dominant(iv) and dominant(v)... Write these down somewhere so you can refer to them, because they will come up a lot in theory of harmony!