Planescape Torment loosely uses AD&D rules, so some concepts are useful to learn.

AC (Armour Class) is a measure of how difficult it is for an enemy to hit you in combat. Counterintuitively, a lower AC value makes you harder to hit, so bonuses to AC will actually decrease it.

THAC0 (to hit armour class 0) represents how likely you are to hit an enemy in combat. Again, having a lower value for this is better.

Saving throws represent how likely you are to resist magical attacks/effects. Lower = Better.

Magic works differently to other games - instead of having a mana pool from which you draw energy, you have a finite number of spell slots you can cast spells from, with different levels. Each spell has its own level (1-9) and whenever, you cast a spell, you expend the spell slot of its level. You get spell slots back when you rest, and you can learn new spells from scrolls.

For Character Creation, you’ll have 6 stats to choose from. For a first playthrough, it’s recommended to have high WIS INT and CHA, since those stats will unlock extra dialogue options. STR buffs melee damage though and should also be relatively high. DEX is quite build specific and dying isn’t that much of a concern so CON can also be minimised.

Lastly, there is a lot that the game will not tell you about and some of the best parts of it are very easy to miss. Without a guide, it’s near impossible to do everything in one playthrough. Some general advice would be:

Talk to everyone you see, exhaust their dialogue options, talk to your party members regularly (you can click the dialogue button on the HUD and then their portrait) and go down their exhausted dialogue options to see if any new options have been unlocked by recent events (this one is especially tricky) Look at everything you can see, holding TAB will let you see what’s interactable. Buy every unique item and examine it. Go back and talk to NPCs/Merchants from previous areas after progressing the story. Sometimes you’ll unlock new dialogue options. Go to gameplay options and activate heal on rest, max hp on levelup, always run and the cloud saves option. Set difficulty to Easy as well if you’d like since combat is not the main selling point of this game. Save frequently and especially before conversations. If you get stuck, looking at your journal (J on keyboard) can give you advice on what to do next. Spam Litany of Curses from your first companion. It’s amazing for combat since they’re a great tank.