Make sure you pick A level physics
Read books on a topic of ohysics that you like
Go to lectures or watch some online on YouTube, I personally liked the ones offered by Stanford.
However, it’s all bit early. I didn’t even know I wanted to physics until the end of year 12 so you’ve got time to build your personal statement to prove your interest and you need to make sure you’re interested in physics and maths at A level otherwise not worth studying it at uni, so there’s still a lot of time to go.