This is wierd i dont know how to back stab yet i have three assins or thiefs in my party

Stand behind your target and draw your blade. Try and find a thin spot on the armour and when its in, move it round a bit to increse your chances of hitting a vital org..

Oh, you mean in the GAME? Sorry.

Stand behind your target while in stealth, and attack normally. Your next attack will be a backstab, but if you miss, be prepared to get the hell out of there.

I don't think you have to be in stealth mode. I've engaged the enemy with tanks than had thief run around behind them then hit with a short sword, short sword of Backstabbing works wonders!

Im pretty sure you have to be hidden in shadows to backstab.. so you need to pump that skill up if you wanna be a good thief (in my opinion). Its worth it.

any type of invisibility will work. However, being "partially visible" with Improved Invisibility will not. With II, you must not be able to be seen, not just be untargetable by spells. Note that backstabs can only be done with one-handed bladed weapons usable by thieves. Thus, a thief with Use Any Item cannot backstab with Carsomyr, and Fighter/thieves cannot backstab with the Storm Star (you CAN backstab with the Staff Mace, though... the only option for thief/cleric backstabs)

Can you use cloke of non detection

cloak of non-detection doesnt work like that. it just prevents an invisibility from going out if someone casts a detect invisibility like spell.(i think)