Use humanity at a bonfire to go human. You can use it again to kindle bonfires, giving you extra flask uses. You can also use it as a clutch heal when you're out of flasks.
In human form, you'll be able to see summon signs from other players/ NPCs, to help with boss fights. But you'll also be open to invasions by other players, and NPC Red Phantoms (phantoms will only invade if the boss of the area is still alive.) You also get bonuses to stats like item discovery, chaos damage, and curse resist, but it's negligible and not really worth worrying about on your first play-through.