Well basically 7th's are a triad + one note.
9th (11th, 13th also) are a dominant chord (major triad with a flat 7) + one note.
If you have a major 7th chord, its a major triad plus the 7th note of the key.

So if in the key of C you have - 1, 3, 5, + 7 - C, E, G, B
C minor would be a minor triad with a flat 7 - 1, flat 3, 5 + flat 7 - C, Eb, G, Bb

A 9th 11th or 13th is the same idea but over a dominant chord.
The 9th = the 2, the 11th = the 4th and the 13th = the 6th. Its like if you where taking them from the next octave (after the 8, being the root)


So C9 would be 1, 3, 5, flat 7 + 9 - C, E, G, Bb, D

There as to be some videos here on GT explaining those Chords. I'm pretty sure by the end of the day Chris will have a bunch of link for you :)