How do I understand what my baby needs and wants?

You won't . There is no way to tell! sorry! you just need to roll with the punches. Just keep in mind that babies cry essentially for 4 reasons... Hungry/thirsty, diaper change, tired and constipated. Once those are covered, the child will likely relax. If the child is more fussy than usual, then that is a sign a child may be sick.Then you need a doctor.
Try. At first, it's mostly trial and error; but you'll get better at learning your own baby's signals. Pay attention to noises baby makes, what they look at, how excited they seem, and how they react to what you're doing for them. You'll get it!