There are definitely good things and bad things about joining mega-websites like Yahoo. I've had mostly good experiences, but I'm quite a bit older than you and pretty experienced at being on the Internet. So I think your concerns are very sensible.
My advice is, ask your parents to help you sign up. Get them to sit beside you and read everything together with you, and decide how much information to give and/or if it's a good idea to join.
You can definitely tell Yahoo that you don't want to receive their newsletter and all that stuff, and they won't send you anything you don't want.