There's really nothing to worry about. We all have dreams about strange, unusual and sometimes scary things. There are a few things you should do before going to bed every night.
1) Don't eat for at least an hour before sleeping
2) No screens (TV, phones, laptops etc.) for an hour before going to bed
3) Try not to eat any dairy products during the day
4) Before sleeping, sit down on your bed and relax. I find that mediating briefly helps. Listen to a meditation CD - I suggest Headspace (https://www.headspace.com)
5) Understand that it won't happen in real life. That's the important part.
What you should do if you are having bad dreams
One thing you could try is talking to someone. Mothers are particually good to talk to, as are grandmothers.
If possible, see a counsellor. You may have one at school, or there may be some free sessions near where you live.
If none of those options work out, you can visit the ChildLine website and talk to one of the counsellors on there. They are very helpful, and will always manage to calm you down (http://www.childline.org.uk). They help any person under the age of 19, anywhere in the world.
Additional information on controlling your dreams
As a brief note, search 'Lucid Dreaming' in Google. Lucid dreaming is a way to control your dreams, and become conscious while dreaming. That might help - give it a go.
You'll be fine, it's very unlikely it means anything. We all get nightmares, and we all hate them. It's just something you have to move past.
