Invest in Vitality for more HP, always useful.
Level up the specific stats that the weapon you are using scales with - there is a letter grading (S,A-D) on the weapon stats page telling you which stats those are. The higher the letter, the better the scaling, the more points you invest in that stat, the more bonus scaling damage you do. So Strength, Skill or both depending on the weapon.
Endurance, yes, but not so much early on as BB is way more lenient with endurance compared to the other Souls games. So I would level up Endurance late game, or when you feel like you don't have enough.
Bloodtinge increases the power of quicksilver bullets (i.e. guns mainly), Arcane is for spells and weapons that scale with arcane. Both less useful if you are going for a pure melee build.
I think the softcap in stats is generally 50.
