You don't need to do STEP to do physics. Only the PAT is required in terms of extra entrance exams.
I got accepted for Oxford physics this year so I'm just going to tell you what I did.
In terms of PAT, I suggest you take a look at some of the past papers now and look at what the questions are like (the earlier you start the better). The PAT is much harder than A-level, so you will likely not understand some questions and will struggle on many. But do the ones you can do and find out what you need to learn in order to understand the hard questions. You will probably need to teach yourself new concepts that you have never been taught. Over the next few months you should try to go through every past paper and you should see your marks increase the throughout the year. When it gets to around 3 months before the PAT (or maybe even earlier), you need to start practicing the PAT past papers REALLY hard. I went through every past paper at least 3 times total. Every time I finished a past paper, I would spend an hour or so correcting my mistakes. By the time you sit the PAT, you should be at the point where you understand every past paper question that has ever been asked. Take a look at the PAT reports to see what score you need to get in. The average offer holder gets about 73/100. I would aim for over 80 marks for a really good chance of admission.
Interviews are the most important part of your application BY FAR. If you do badly in your interview, you will probably get rejected, and if you do well, you will probably get accepted. It's as simple as that. I recommend not worrying about interviews until you do your PAT, because PAT preparation will indirectly prepare you for interviews anyway. However, after doing the PAT, don't think the hard work is over because the interviews are exactly twice as important to your application than the PAT (this is stated in the PAT reports). So, after the PAT you should be doing interview practice as much as possible.
You don't need to do any super curricular activities, but it might be worth putting them on your personal statement if you do. I recommend doing a physics Olympiad or maths challenge. There are also summer schools you can attend which shows you are interested in the subject. When it comes to super curriculars it not so important as to what you do but what you say about it. E.g. nobody cares if you've read a book on quantum mechanics, but they will be impressed if you can show that you've understood a complex topic from a book. I recommend reading a book on physics that explains hard concepts and writing about it in your personal statement (only if you understand it). I did mine on quantum mechanics because it made me sound smart. P.s you don't need to read a book with difficult maths, just read an interesting book that goes beyond what you're learning at school. Go for quality over quantity when it comes to super curriculars. It's more impressive to extremely well versed on one topic then it is to know a little about everything.
Overall the most important thing by far is interviews, but they are far away so you need to focus on the PAT for now. I wouldn't say you need to start now (I would start by April at the latest), but if you start working on the PAT around this time of year you will probably get into Oxford. For PAT practice, do past papers, teach yourself the syllabus (which is given on the Oxford website), and practice with other resources such as
https://i-want-to-study-engineering.org/.