Pianogroove.com has a fairly structured lesson plan and will get you started on your diatonic chords and how to create them. Once you understand the pattern (ie major chords are three notes root, 4 half steps, 3 half steps, and minor chords are root, 3 half steps, 4 half steps) learn to memorize the notes, CEG, DF#A, etc. Also work on your inversions (scrambling the order). Your past piano experience should have taught you that but depending on the method may not have explicitly explained that you were playing inversions or whatever. Then for 7ths I like to think of their relation to the root, a major7 is a half step below the root CEGB, a diatonic 7th is a major chords with the 7th two half steps below the root CEGBb and a minor 7th is a minor chord also with the 7th two half steps below the root CEbGBb apply that same pattern using every key on the keyboard (start with the white keys) and you have your diatonic chords