How do credit card work?

This is probably a simple question for most of you here, but i have no idea. Im 17 and all my life Ive been running from becoming older. Im trying to learn about the things I dont know but will need.

Credit cards are one of them. How do they work? What is interest? What does it mean when someone says “pay back the balance” ? do you have to add up all the money you used on your card that month and pay it all back? What if you earned the money and its not a loan? What the fuck is credit score?

are there any classes i can take that will help me because i feel so stupid for not knowing the most basic stuff