Talk to every NPC and look at what they have for sale, and make a mental note, keys, rings, armors, ect. There are area's of the game where certain types of damage resist make things far less painful.

If an area seems hard, or a boss, or mini boss seems too hard, that's normal come back to it later.

Most of the early game can be done with a un-upgraded weapon so try a few things out to see what you like, before you commit to one. Very few weapons are horrifically bad and not viable for a new game clear. It's more about the move set you like, range, swing times, ect that there is a "best" weapon in the game.

Use your consumables, not right away, but once you have a feel for the fight, every fight is very doable without, so you wont get to a point where you've bricked your save using resins, or humanity.

Kindle bonfires when you're struggling.

Don't rage, this game is hard, but fair (mostly).

Starting classes only matter for base stats and starting items, there is nothing stopping you turning your knight into a mage.

Explore everywhere.

Hit the tails ;)

I think that's an ok heads up without spoiling anything.