hey man,
once you already know the major/minor scales, learn harmonic and melodic minor. it is VERY helpful to learn
intervals but this is usually a pitfall for most people...it just takes time and practice. I literally used flash cards and drilled the shit out of myself to learn these...then would later bring it to the keyboard, every.single.day.--- during that time you will learn the concepts of augmented and diminished qualities. It's sink or swim and I knew i was gonna swim.
from there, it is then easy and helpful to learn basic chords; then progress from there. this is going to take a lot of practice...i am not aware of an easy way to learn it without practice and work.
there are many places online where you can learn this stuff, I recommend browsing around and seeing which presentation is easiest for you.
when I was initially learning (and still am learning) i find free sites with worksheets that allow me to write it out on a staff (check my answers) and practice at the same time. this way I have a visual accompaniment to allow further understanding of what is going on musically.
after that, consider looking into fifth chords and minor Major seventh chords. they will assist in traversing chords from one key to another, while keeping the listener intrigued.
i have also found it helpful to look into specific classical music examples of how some of the greats have done it. learn from the works of those who wrote great pieces of work.
keep at it. it takes work.
