Hey
It would be a good idea to do your best in your GCSEs, and then A levels. (If you live in the UK). If your grades are good at A level you could go into the Army, Navy or RAF as an Officer. Also, they can sponsor you through university if you want to get a degree.
However you don't have to have a degree. But it is a good idea to at least try and get some A levels, as this would open up many doors for you after you leave the Armed Forces or if you decide not to join them or don't pass the various interviews and basic training.
I don't know about the Army, but the RAF don't allow women into the RAF Regiment, the RAF's land army who protect the bases and such. Women aren't allowed to fight on the front line by law, for some reason.
You can join any Armed Force as a rate with no qualifications, but you would be doing GCSEs anyway if you are a brit.
The Armed Forces is a good career path, but only if it suits you. It tends to be very rewarding. Your parents may not totally agree with it but in my opinion, it would be wrong for them to limit your careers to traditionally "girly" things. Being in the Armed Forces isn't a "boyish" thing, there are loads of women in our forces and they do a damn good job!
It's perfectly normal to be interested in these things, and you are not too young for it. It's getting near the time where you need to decide.
I'm in a bit of a pickle at the moment! I have to decide what I want to do. My A level maths exam was nothing short of a farce which has closed the doors on what I wanted to do previously. Looking at some other degrees up north in London now!
Hope I helped you in some way, feel free to pm me.
Check out these links:
http://www.aircadets.org -- The Air Training Corps website, you may be interested in it
http://www.armycadets.com -- The Army Cadet Force website
http://www.armyjobs.mod.uk -- Nobody other than the Army
http://www.royal-navy.mod.uk -- The RN
http://www.raf.mod.uk/careers -- RAF Careers
Hopefully they will be of some use. Also it may be a good idea to pop into your local AFCO (Armed Forces Career Office) where you can get loads more info on Armed Forces careers.
Cheers,
Mark