How do you backstab an enemy?

I know you have to be behind the enemy but is there a key you have to press or do you have to use the script?

I can easily get behind enemies without being detected but then i am lost, my thief is lvl 12 so i would like to be able to backstab someone soon.

You have to be invisible for it to happen. Either make your thief "Hide in Shadows" or let him gulp down an invisibility potion. If you then let your character walk around to the creature/person you want to backstab, and he performs a successfull hit, then you'll have your backstab.

But how do you 'hit' the enemy? is it just the normal attack option?

yes, just go invisible, stand in the back of your enemy, then attack. with your lvl 12 thief now, when it succeeds, you're going to enjoy the feedback message very much :D

Proficiency matters as well of course; 3 stars in short swords helps a lot ;)

The only problem with Thieves is their lousy THACO and that`s the reason why they miss target so many times. But when they do hit they rock.

So that`s why I like F/T with Quietus and Critical Strike. (ToB)

You still get attack bonuses when you're hidden in shadows. +2 to hit IIRC.

Do bless, chant or luck buffs help in anyway with the chance of backstabing?

I'm playing with my thief that I ran through BG 1 and converted to an assassin for BG 2, and I have abosolutely no trouble hitting my targets.

And anything that gives you bonuses to hit will help a backstab.

They'll increase the chance that you hit. You're making an attack roll.

BTW the said attack roll is made at a +4 to hit (at least it is in P&P D&D).