I think forensic pathologists are usually medical doctors so you would need to study medicine at University. That would mean you need to study some science A Levels (I think the exact requirement varies between different medical schools) and get good grades in them.
That’s fantastic, what an facinating subject! I’d read as much as you can about it and learn some basic principles, which always impress in your entrance interview for univeristy.
I think David’s right, at the moment you’d have to become a medical doctor first, then have further training so it will take a while from studying to qualifying so be patient. There are other type of pathologists that takes less time and do not require a medical degree, you can get more info here: http://www.rcpath.org/index.asp?PageID=1381
The best advice I can give is get lots of experience, jobs in forensic pathology are quite rare and the subject is increasing in popularity. The best way to make yourself appealing to university courses and future employers is to get experience. So this can be unpaid, voluntary work in labs or spending your spare time doing something related to the subject. All of which will look good on a CV or a university application 🙂
Find out exactly what qualifications you will need and try to find out as much as possible about what forensic pathologists really do, so that you know what you’re aiming for and what to expect when you get there. Try to get some experience, either in a forensic pathology lab or somewhere similar. This will demonstrate you commitment and willingness to learn to universities/potential employers and you may also pick up useful contacts and skills for the future. Best of luck!