Trigonometry, Pythagoras' theorem, sine cosine and everything shape related are often close to the back of the paper. Algebra is ALWAYS, and when I say ALWAYS, I mean it. There is at least one full page of Algebra questions eg. expanding brackets, factorising and even indices are on there most of the time. Some questions even have certain topics mixed in one. Stratified sampling, percentages, scatter graphs, cumulative frequency, ratio etc. I can't list everything but those were the topics that came up last exam.
I would say the higher paper is pretty easy to achieve a C or above. I sat my first ever GCSE paper in June, which was the Edexcel higher maths and when I sat that I was in Year 9, and I got a B. I'm pretty damn sure you'll get well above a C. Good luck to you.
If you look through the maths section, I remember someone posting last year's paper on there.