Omg please help me!!!!!! please im so scarred?

Ok so i have a dentist appt. Tomorrow and im rlly scared because im my front molar there is a big chunk missing out of that tooth. I think there was a filling there but at like the top of my tooth there it a small amount of gray. It is still my baby tooth too. Also my back molar (havent gotton my 12 yr old molar yet) has a small black hole too. Im so scarred plz help me! And they dont hurt. P.s wht is a root canal and how do you know if you need one.