First of all, good luck in getting your dream career
There are several options - seeing as you're still at school, I'd say the best bet is to choose subjects for A Levels that will let you go to university to study Mechanical Engineering. These are maths, physics, and design. Becoming an engineer will mean that you learn how to design, test, build and develop F1 cars, road cars, planes, or anything along those lines. Make sure to go to a uni that has a 'Formula Student' team, as this will be a brilliant way to gain experience relevant to F1. If you do well, you can work with an F1 team (or any motorsport company) for a year, and then you'll go back to uni to do your final year of study.
That's what I've done, and the future looks bright! There are other ways, but I'll leave that up to other people to explain, as they'll know more about it than me. Again, good luck mate.