THAC0, for the record, is "To Hit Armor Class 0", meaning that you need to roll that number on a D20 to hit someone with an Armor Class of 0. Positive Armor class subtracts that number from your THAC0 every time you attack someone, and negative adds to the THAC0. 2nd Edition is weird, you never know if a positive number is good or bad.

By the time you hit level 2 (which isn't long), everyone besides the wizard should be a lot less squishy (and the wizard shouldn't be anywhere near the front anyway). However, this much better handled in 3.5, especially since you get max HP at the first level.