It's easiest to start out as War Veteran, most straightforward playstyle (just keep in mind that other classes take a turn for switching weapons).
You get XP by learning of new enemies/items rather than kills, most enemies are only worth killing if they're an obstacle to somewhere you need to be and you can't run around and block the path behind you (doors can be jammed if you have spikes, just press to close the door an extra time after you've already closed it).
You can't spend forever at one depth, you need to budget your mental stability to last you to the stairs downwards.
Potions with negative effects aren't useless, since you can throw them at enemies.
If you don't have a numpad you can move diagonally with Right/Left arrow +Shift for up, +Ctrl for down, but it's much more comfortable to play on numpad. Check the F1 manual for other keys, you'll need most of them.
Undead corpses rise again if you don't destroy them, kick them with same key you kick down doors with.