Hi,
I'm guessing you are doing the 9-1 paper, and I'm doing Edexcel also.
The topics that seem to come up most in these papers are: Graphs (Histograms, etc.), Geometry (circle theorems, Pythagoras, etc), Indices (negative, fraction, etc.), Ratio (different problem solving questions) & Probability.
I'd say ratio and probability come up A LOT and have the broadest set of questions, so practice as much questions as you can get your hands on. Geometry also takes up HALF the paper, so if you want to pick up the biggest set of marks, PRACTICE THOSE!!
But you really cannot study these topics alone - they rely on you having knowledge of how to do easier work (basic knowledge) like fraction work, how to expand and multiply algebra brackets, algebraic fractions, quadratic formulas, surds etc...
So I would suggest to study the 5 topics I mentioned, but do make sure you have your basic knowledge covered, and practice many exam questions on these topics because they mix them. For example, a rectangle with Surds in, circle diagrams where you will have to be strong on algebra, ratios and circles together....
Also, I'm not sure about how much you'd need to get on each paper, as with this new 9-1 scheme the grade boundaries are based on how everyone in the country has done, so it fluctuates, there are no set boundaries as with the previous A*-E I believe.