how do you backstab? i know you must hide your thief but then what? just attack o do you need to hit a special key? also do you need to have a certain type of wepon or be behind them? thanks for the help!

In order to successfully backstab, your thief must either be hidden in shadows or invisible, behind the target, and using a short sword, dagger, or katana. No special key is required, just attack as normal.

awsome! thanks for the help :)

Isn't longsword ok too?

Thieves rule, good fun :) Here is a little overview on thieves with a little guide to backstabbing at the end:
http://www.sorcerers.net/Games/BG2/thieves.htm

I'm quite sure that you can backstab with all melee weapons a thief can use, including staves and clubs.
You cannot, however, backstab with the staff of the Magi. So don't even think about it! ;)