The boring option, but one that many of us who are now old farts would love to go back and do all over again.....is to work and study as hard as you can while you have the time, freedom and opportunity.
That's the boring option, you're 15 and that will go in one ear and straight out the other, as it did with me when I was 15.
The only thing I will say in favour of study, is the more academically 'rounded' you are when you hit that job market, the much better chance of getting the job that you want, and that includes getting into something like drama school.
In the meantime, go and talk to your drama teacher at school, if you have one. Show your interest, and volunteer yourself for anything going in that department.
When you are a famous actress, you can thank me by getting me a part in one of your films.
I've always wanted to play the grumpy old Concierge in a posh hotel.