The only things you really need to know are that lower ACs are better than higher ones, lower THAC0s are better than higher ones, and lower saving throws are better than higher ones. You also have to prepare all of your spells in advance, and when you cast a spell, it's "used up" until you rest. You can know a wide variety of spells, but you have to choose which ones you're planning on using beforehand. (You can memorize a single spell multiple times, though.)
That's really all you need to know, honestly, and everything is very well explained both in the manual and in the game itself. The magic system in particular is explained much better in PS:T than in most other D&D games. PS:T is pretty light on game mechanics though, most of the game is done through dialog.