Chords can be very simple, but they took me a hell of a long time to figure out!
I found
www.jazzcenter.org was really useful, esp the section 'Chord Symbols and What to Make of them.'
A lot of the stuff you might not need to know right away. Pay the most attention to Major 7th, Minor 7th, and Dominant chords. You'll see what I mean when you check it out.
The first thing I did was to play these chords just as they appear in all the keys. For example, for Cmaj7, I'd play C, E, G, B. Those are the basic notes associated with the chord. That's what the site essentially tells you. Get this so it's pretty much second nature. You see the chord, you play the notes associated with it.
Once I got the feeling as to what they look like, I played them behind some tunes I got out of a fake book. Now I knew how they sounded and had a basic level of profinciency.
Only thing is, nobody actually plays Cmaj7 with C, E, G, and B. They mix these notes around on the piano, they add other notes and so-on. Although Cmaj7 may consist of C, E, G, and B, the 'voicing,' i.e. the way you actually play it, could look quite different. For that I think you'll need some kind of teacher, as it can be a bit more tricky. This should at least get your foot in the door.
By the way, it is good the check a lot of jazz resources. The reason is that jazz revolves largely around the idea of chords. Even if you don't want to play jazz, studying some its theory can be very helpful no matter what you play.
