Many people have trouble with bullies. If you have a bully you should report this to your principle. No one should have to deal with a bully. Besides now a days some teachers be to scared to do anything when they see someone being pick on. If the principle don't do anything have your parents go to the school and talk with the principle.
It's important not to keep this to yourself. You need to report the bully, make sure to tell your family about what's going on and look up the school rules/policy on bullying so you know where you stand and if the school is breaking their own rules by not dealing with the bullies. If your school has any sort of mentor or guidance counciller for students to talk to if they have questions or troubles you should also go to them.
Tell your parents, tell your school administrators, and try telling the bully to back off. If things get at all physical, you should call the police immediately!!!!
Remember, school is temporary. And, this bully is just a temporary part of your life. Don't let her have a more permanent place in your heart and mind than she deserves. I know you like Demi Lovato, and you can see how bullying affected her, and now she is dealing with it later in life. Don't let that be you! Learn from her example, and seek counseling if you think it will help.
I have to agree with all the answers that say to tell the principal and your parents. This is especially important if you think she's dangerous and will physically hurt you.
But, if this is strictly about being picked on verbally, there's another important thing you have to do: keep your self esteem high. Remember that you're smarter than this girl, because you're not a bully. Remember that her poop stinks like anybody else's. Hopefully you've got family and friends to remind you of everything you've got going for you. And remaining mature in the face of a bully, either by ignoring her or reacting calmly, will make her feel like an idiot. Good luck.