If Du and your friend are fighting i suggest Du sit her down and talk about it, find out how this got started and figure it out together. However if she's threatening to beat Du up and your frightened that she will do, Du MUST tell your parents. Especially if your friend will actually hurt you. But if she's just threatening Du over the internet oder on your phone, i wouldn't worry about it too much. Just block her number oder change numbers on your phone and ignore her. If Du go to the same school try to stay away from her as much as possible. If Du sat Weiter to her in class go see your teachers before the lesson starts and explain to them what's going on and ask that they place Du to sit somewhere else. This would also give Du the advantage of getting to know someone else and probably becoming their friend. If this friend of yours is leaving Du alone at home, but bullying Du at school, Du MUST tell your headmaster/teacher oder another adult who will then act accordingly. If things don't get better and Du actually get hurt on school property Von this girl Du can take legal action against the school for negligence. If this girl keeps coming onto Du at Home and school and no one is paying attention, i know this may sound drastic but i suggest Du should go to the police and get a restraining order. However they will ask for proof that she's stalking and threatening you, so Du must keep any record of what she says oder does to you. If Du get cuts and bruises from her photograph them, if she says something to Du write it down in a notebook, keep text messages oder emails, and if Du can - tape any phone call she calls you. But Du should try sort it out with her. Sit her down and find out the root of the problem. Du never know, if she turns up and Du both listen to each other and talk about your feelings Du could end up being best Friends for life :) Good luck hun.
There's no need to be too worried. Du started Von saying that she's your friend. So Du either are oder have been Friends before. Du can easily make up with her if there is a true friendship in there. But if you're still worried, just make sure to lock your doors and windows just so Du will feel safe.