If you do get spotted or heard by a guard, don't try to take him on in combat unless you are especially confident.
Your best bet to run for the nearest large shadow, crouch and try to hide. If you have any equipment, such as
flashbombs or crack arrows, these can be used to disable the guard while you make your escape.
Often guards will place mines, whistlers and caltrops as traps at important objective locations. You can use
your water arrows to disable whistlers, moss arrows to neutralise the caltrops and your lockpicks to disable the mines.
Learn the maps to discover the locations of the loot and key items, the most shadey routes and the safest way to the exit.
The blackjack is a powerful weapon that can knock out guards instantly. Watch a guard to see his movements
and then strike quickly, hitting him in the back of the head. Use the heavy attack if he's wearing a helmet, and with
luck, you'll see him collapse on the floor.
As a thief, your objective is usually to sneak into a guarded place, steal some loot and items, then escape.
You must remember that a thief is physically weaker than a guard and has less offensive equipment. To succeed,
cunning is required. Move slowly, stay in the shadows and don't make ANY kind of noise if a human guard is nearby.
Put out torches with your water arrows to make the level darker and give yourself more places to hide.
Crouching and moving slowly will make you less visible, keep an eye on the lightgem at the bottom of the screen,
when it is totally dark, you are invisible. Drawing weapons will both make a noise and make yourself more
visible, so don't do this if you're trying to hide. 
Items - all the items available for the thieves.