Hear & play major and minortriadmelodies — adding the 3rd to the root & 5th, the one note that makes a chord major or minor. Major and minor triads are the most common, consonant, and versatile chords in music, so this is where melody and harmony first meet. By the end, you can hear and place either triad from any root.
Why
The triad is the home base of harmony — major and minor are the two chords everything else extends from, and they are everywhere
Choosing their notes intentionally gives you a reliable anchor in solos, because the chord tones sound especially consonant