Here's a few things I wish I understood when I picked up the game, which I think are great to know.
Weapon accuracy: For most melee weapons in most situations, Strength determines their accuracy. Melee weapons with the Finesse feature will use the highest of your Strength or Dexterity. Ranged weapons always use your Dexterity.
Equipment Proficiency: It's generally not a good idea to use equipment that a character is not proficient with, because you get penalties like being much less accurate with weapon attacks or be prevented from casting in armor you're not proficient with. There are exceptions, but for now it's going to be a lot less headache to just avoid using equipment you see the proficiency warning.
Dexterity: No matter what class a character is, it's helpful to have higher Dex. This means better chances of acting first in battle(this is extremely powerful), avoiding damage from spells, avoiding certain spell effects, making ranged and Finesse weapons more powerful, and increasing your Armor Class(higher AC=lower chances of getting hit by weapons). I generally run my whole party at 16 Dex, except the ones attacking with it I try to have higher.
