I don't know . .

Hi. I just need to get something off my chest.
I have this ffriend at school, she makes fun off me when I try and talk about something and when I try and give my opinion on something, she excludes me from things, she makes fun off the things I like and she just has a go on me for no reason at all!
I don't know how to stand up to her, she is a lot bigger and louder than I am and she's a lot more confident! It just really brings me down and I don't know what to do! If I did tell her anything she wouldn't belive me and have a go at me!
Sorry I just don't know what to do!