My hair is really thick too. Um, I hated it when I was littler because it seemed to get dirtier easier.
If your parents will let you, buy a straightener. Even if your hair is pretty straight it helps to make it not as big. It also reduces fly-aways and smooths it.
I would get it cut, long hair is more for thinner hair.
You can have them thin it out. It makes it look thinner and it's nice for styling.
Also get layers. I <3 layers.
Then find hairstyles that help control its thick-ness.
Get a long barette like this one:
Barette on Flickr - Photo Sharing!
I prefer black or brown ones, but you can get ones like that with pretty patterns.
Then you pull back the hair half an inch or so and above the ear back and secure it with the barette. I leave it so you can still see the part because it looks messy but really cute.