And also how in the heck is my thief suppose to backstab? Every time I attack a monster from behind they turn around.
You need to have a different character attack the enemy first in that round. That establishes the enemy's facing. Then you can attack with your thief from the opposite side of the first attacker. Make sure that your thief is not wearing any armour heavier than leather (if multiclassed).
I agree with what kmonster said above, and to add a few more clarifications:
1. There are often only a limited number of random encounters in an area (usually 10-15), once you defeat all of them there won't be any more in that area and you can rest safely.
2. You can see the movement rate on the character sheet to note how far it's gone down.
3. Some locations on the maps will note "This looks like a safe place to rest", you can rest in those areas without risk of being interrupted by a random encounter.
Another tip: always have at least one Detect Magic memorised, and always cast it on the loot following a battle. (There is no other way of finding out if any of the items are magical). You can skip this for wandering monsters in the early part of the game though, they never have magic.
[Minor SPOILER ahead: TheOnlyRandomEncountersWhichHaveMagicItemsAreNPCFightersClericsMagesandThieves)