Leveling guide?

Either follow the main quest (started at the crash site) or just wander around and do stuff. The main source of xp are (sadly?) monsters which you kill, and compared to wiping a dungeon, doing quests doesn't give all that much exp. The quests are there rather for the purpose of character creation, setting your alignment and determining the outcome of the game (described in the slides once you complete the game).

So if you want to get levels, try taking master level quests, visiting sewers, dungeons and other monster infested locations. The character level, in practice, only determines how much hp/fatigue you have, everything else depends on how you've spent cp's. If you're a fighter with no charisma, even at level 50 you won't be able to do social quests, also with no combat skills, you might die while killing pigs in ashbury even if you're at level 30.

I recall there being some kind of a huge Arcanum FAQ/walkthrough, which listed that (along with details on how to complete the quest, which you're not forced to read). There's an Arcanum wiki, but it's not very thorough yet.

It doesn't scale. I think random encounters are scaled, or can be scaled, but otherwise everything is quite the same. However, the followers' starting levels don't raise to catch up with yours, so unless you want to walk around with a bunch of weaklings, better get them once they're your level.

 

I mean the ones that interrupt world map travel - I think the ones with assassins (the hand, bates) are more or less scaled according to your level.

However, there is practically no noticeable scaling present in the game - the assassins you will fight are still quite easy prey for any kind of a combat-aligned character.