...unless Tina is a troll. If not, apologies, but Leah, as others have said here, be careful about what details you give to people you don't actually know. Someone who says they are also a 13-year old girl and learning the language might actually be a 50-year old bloke trying to get your contact details for rather more sinister reasons.
Anyway, if, as you say, you are half-German, then surely your German parent must have some relatives or friends in Germany and could maybe organise a holiday trip for you to stay with some of them, or invite a teenage cousin from Germany to come over and stay with you for a while?
Also, depending on where you live, as people have said, your local university might be able to put you in touch with German students. Most towns also have twin towns in other countries.
And there are quite a lot of Germans who live in the UK and might be happy to help you for a smaller fee than you would pay in a proper language school. Again, though, please be careful - I would probably ask my parents to help check these things out, just to be on the safe side.