Well, you're on the right trac. The A you played was an Aadd9. It's called an add9 because you put the 9 in over the top of a major triad, without adding in any note between. Normally, extensions are built of a dominant 7 chord - a major triad with a flattened 7. In the case of A, an A7 would be, A, C#, E, G. With an A9, you should put both the 7th and the 9th in. Therefore, an A9 would be soemthing like this:
E---7---
B---8---
G---6---
D---7---
This is the same for all extensions. Usually, you are supposed to add every extension below the one notated, so an A13 shouls have the b7th, 9th, 11th and 13th. The limitation is that a guitar can only play so many notes, you have to choose what ones to play. The fifth is usually the least important noe in a chord, so that can often be dropped, unless it is the melody note. Jazz players will often drop the root as well, but your band mates might look at you a little funny if you did that. The third is very important, as is the extension notated, and the 7th. Otherwise, have fun making up chord shapes. There are some common shapes though that you would be smart to learn. AN example is a 13 chord. Here is an A13:
E-------
B---7--
G---6--
D---5--
A---X--
E---5--
This has the root, third, seventh and thirteenth.
YOu could put the 9th in by flattening your pinky and playing B on the 7th fret of the E string, but generally shouldn't put a note higher than the extension (other than the seventh) written.