After school, you need to study science at university, and then do a PhD. It’s also possible to get a job as a scientist straight after university and work you way up from there, and sometimes you can do a PhD as part of your job.
By studying well and getting good results in your GCSEs and A-levels (in science subjects). You should also try and develop an interest and knowledge of current science developments, perhaps by reading New Scientist or listening to science podcasts. From your A-levels you should get a degree in the science topic of your choice (mine was Physics) and from there, perhaps an MSc. and PhD… And then a job in perhaps a university, or maybe some science-based company.