What should a 15 year old be doing to become an astronaut?

Okay, here's the thing - I really Really REALLY want to become an astronaut. I've wanted to since i was tiny - my first word was 'moon'. Its all i can think about. I do very well in my sciences (i got 100% for my entire science GCSE) and pretty good in maths (i'm predicted an a* for gcse). At A level i want to take Physics, Biology, Chemistry and Maths. After this, i want to go to university - but here's the thing - what course should i take? I know the ESA website says physics, but there are so many physics courses! should i take physics with astrophysics? space systems? engineering?... please help! Also, is there anything in the meantime i should be doing?