It is a bit confusing at first.
Basically:
- World Tendency is effected by dying in human form, killing bosses, NPCs or Primeval Demons (these only show up in black world tendency)
- Character Tendency is effected by invading, being invaded (including NPCs) or killing NPCs
http://demonssouls.wikidot.com/world-tendency
http://demonssouls.wikidot.com/character-tendency
So if you want to keep white world tendency for example, never die in human form in a world. After defeating a boss (and regaining human form) return to the Nexus, jump off a ledge to die, then return to the world, it will now be a step closer to Pure White (PW). You have to leave a world for tendency changes to take effect. So to go full black world for example, kill yourself 7-ish times in a world, each time using a Stone of Ephemeral Eye to turn human, return to the Nexus, return to the world and it will now by Pure Black World Tendency (PBWT)
It is possible to do all world and character tendency events, white and black, in a single playthrough but it's best done with a guide or a checklist. It does all sound confusing, but once you've got your head around it, it's quite easy to do as long as you are careful - I always have a little checklist at the side of me when doing all the events, kind of old school!