Ok so I need a lesson on Chord Building, Im pretty good at comprehending things when it comes to theory, but I need some things cleared up here. Btw I know how to do it, but I need help transfering it to guitar. So I know how to build triads. Lets use a G chord as an example for this. A G triad is:

D
B
G

A guess a G seventh chord would be

#F
D
B
G

And so forth. Now on guitar, to play a G Major chord I would play the

E String: 3rd Fret (G)
A String: 2nd Fret (B)
e String: 3rd fret (G) (This confuses me I didnt realize this untill now)

But there is also all the Open strings (D)(G)(B)

Thats 6 notes...Which would be an 11th chord?

But anyway, can someone explain how to play a G Triad, G Seventh, G Ninth, etc and why.

its still a triad because your only playing 3 notes (gbd)
there are several different ways you can play those chords, open or closed with several places on the neck. just make sure oyu have at least one of each note you need

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So the 3 other string simply dont count?

in a way, it matters as to how its voiced but not the type you could have
F#
G
G
B
D
F#
G

and it would still only be a G7 because its only made up of GBDF#

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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