Which subjects do I need to study in order to be an actress?

Being a professional actress is not like being a doctor, where you go to pre-med then med school, get a residency and internship at a hospital, take an exam and if all goes well you are a doctor. There is no "career path" to becoming a professional actress. It is simply not a sure thing that you will ever actually earn a living as a professional actress. However, there are things you can do to increase the chances of becoming a professional actress.

At the top of the list of things is training. Not just acting training, but also training in voice, singing, dancing (ballet, tap, jazz, ballroom, etc.,) music and other perfoming arts. Early in a career, aspiring actresses never know where their first jobs will come from. An actress who is a competent actress but doesn't know how to sing or dance will have far, fewer job offers than a "triple threat" actress who is a trained actress, singer and dancer. Being reasonably competent in as many of the performing arts as possible will make an actress much more likely to get professional work, be it in the theatre, TV, commercials, radio, theme parks, cruise ships, touring shows, voice-over, trade shows, etc.

At this stage in your development as an actress, I suggest you take as many music, dance, voice, singing, etc. classes as you can in your school or in workshops or classes outside of your school. If there are no acting classes available, don't worry about it. You can start your own performance group (either as a solo performer or with other like-minded folks in your area) and arrange performances at local venues. This will be great experience for you and you will learn a lot. It will also teach you how to get your butt out there in front of an audience and get comfortable with performing - which is what actresses do for a living.

If it is financially possible, check into attending an acting/performing arts summer camp to get additional training and performance experience - http://www.redbirdstudio.com/AWOL/camps.html.

Study hard and get good grades in school because that will give you an opportunity to get into a college with a good actor training program (start researching those now so you can be ready to apply when the time comes.) If you do all that, you will have a good start