The only thing I wish I had known when starting: when you beat big demons (bosses), you'll get a special named soul, like Iron Demon's Soul, it's like an item. You can go to your inventory and "use" them to gain a few thousand souls, but DON'T! Save them for later, they have other (better) uses. That's it.
Everything else, treat it like an adventure. It's a role-playing game. Role-play. You are thrown into this world and you must figure it out. Many details of the game's mechanics go completely unexplained, and trying to figure that stuff out is a big part of the experience. Dive in.
And yeah, as others have mentioned, if you find the game too challenging, then chances are you're playing too aggressively. If you play cautiously you shouldn't have too much trouble.
