This lesson is on chords, how they work, and the basic intervals that make them up. Learning the underlying music theory behind chords will not only allow you to find any chord you want, anywhere you want, it will also give you a solid foundation to build your entire understanding of music theory on.
I have a bachelor's in music (I took about a billion theory courses), and I'm a full time music teacher. After trying to help so many people learn music theory, I've decided that this is the best, most useful and most easily understood way into music theory. You don't need to know anything about music to get started on this, other than the names of the notes (and if you don't know that then google it; it's cake). Have fun.