Pick whoever you want to play the most. Whoever you have the most fun with is going to be the best choice, and you'll be more likely to enjoy yourself much more. Try attending some tournaments if you can, it's easily the best and most effective way to learn. Don't worry about being "too young", the Smash community is very accepting of people of all ages! I'm one of the younger members of my community myself, though I'm 17 which is much older than 12. You'll learn very quickly given your age though, so it shouldn't take too long for you to get good at the game. As for character specific tips, check the character boards for each character, join the discussions, or just read up on some info. Smashboards is a great place to learn, so look around, you'll find something.