Zepplin,

The triads are created by stacking thirds...

1 C D E F G A B
3 E F G A B C D
3 G A B C D E F

To creat a 7th chords you just stack on one more third. The G7 is the dominant seven because in the key of "G" the F would be sharp...

C D E F G A B
E F G A B C D
G A B C D E F
B C D E F G A

Likewise a 9 chord just stacks on one more...

C D E F G A B
E F G A B C D
G A B C D E F
B C D E F G A
D E F G A B C

You do the same thing to create 11th and 13th chords but they get a bit more trickier to play. I think to ask your origninal question the 11th and 13th chords need to be an octave higher. That's technically speaking but people sometimes play with the rules.

I hope this helps,

Manx