If you follow past papers back c. 10 years, attempt the questions and learn how to approach the questions you can't do, you're more or less guaranteed a decent grade.A lot of questions come up time and time again with different numbers/variables/etc. so knowing how to do all of the questions previously set is very useful.
They will put new stuff on the exam (often interesting twists to make conventional questions a little more difficult) and they do depend on some level of mathematical ability but for most, at least some are pretty manageable and even if you get all of them wrong, unless the exam paper you're sitting is particularly mean, you can still get a pretty good grade.
An A at GCSE is a decent start for A-Level. My teacher used to say that in some sense A-Level was easier because you could be sure exactly what topics would come up (pretty much all of them) whereas for GCSE, it is a guessing game.