It will depends on your school, as some are much more prescriptive than others. But... The legal requirement for what you legally MUST cover in each key stage is set out in the NC document. Then you need to know how much of that is covered elsewhere, in other year groups in your school, making sure you 'plug any gaps and repeat areas that need to be taught every year. Then, if you're lucky, you might get some say about actual topics. Personally, I would choose these based on my own expertise or interest, as I will probably teach those things better. Otherwise, you might teach areas for which there are lots of existign resources, or you might follow a commercial scheme of work.