You don't need to start at the start of Year 10, definitely not. (of course if you can then go ahead!)
Most of the good, model, enthusiastic and motivated students would start their revision at the start of year 11 the earliest (I only know one person who did it, and she got 8A*s) and majority of model students would start revision before mocks in January. Some bad students would start from Easter holiday in Year 11 or even worse guys start a few days before.
I would say make sure you have your notes from class are organised, and make sure to pay attention in every class and do every homework given. That should be enough for Year 10, really.
If your class notes are organised, you may not even need to make hundreds of pages of revision notes later unlike some people spend hours on. You could just do a simple timeline or spider diagrams for each topic. Listening in class is very essential as well, though it can be boring, especially for people who learn a lot by listening (like me). In my case, most of the concepts sink in my brain just by paying attention in class and interacting with the teacher and I can hear the teacher talking in exams (ofc they don't but it feels like it).
You can start revising starting from year 10, and it's good that you are a serious student, but it's very hard to keep yourself motivated for GCSEs they are really dull imo...you are forced to take subjects you don't care about and even if the subject is your favourite it can be too easy sometimes or maybe too hard possibly.
So yeah as I said if you keep yourself up to date with all the homework and classnotes, pay attention in class then you are fine!