Get your teeth professionally cleaned first by a dental hygienist. You probably have a significant amount of surface staining after all these years. It's possible that the hygienist may be able to clean them and you may be satisfied with the shade from this alone. If not, then they can recommend a whitening product.
Look in the yellow pages and call some dentists and see if they can direct you to some free dental care. Since you are still considered a child, they may be able to help you. I am sorry you are going through this.
I know I said to look in the yellow pages, which I don't know if you have, maybe a better idea would be to go online if you have access the internet. Maybe even a counselor at school could help you.
Thanks for your help and suggestions.
I am hopefully going to go to the Densist on Saturday and hopefully my mum takes me because this is really annoying!! :(
I don't think I could talk to a counselor, it's too embarrassing to tell a stranger..
I have Yellowpages, but what would I need to seach for on the internet?
Try natural teeth whitening products like lemon skin and a mixture of salt and baking soda go to google and search: natural teeth whitening.