It's impossible to predict how difficult you'll find A-Level maths.
For me, I found it relatively easy, but did struggle with a few things (damn you, vectors!). It's mainly about the work you put in, I think - the workload is a big step-up from GCSE, and most of my homework came from maths.
I found C1 really easy, but nothing new was introduced to me there as I did the FSMQ so had already covered integration and differentiation. Was quite nice to start off A-Levels with a doss module.
I can't actually remember much of C2, I just remember I didn't like it, for whatever reason.
C3 and C4 were probably theoretically harder, but I'd say the C3 exam was trickier than C4; C4 was, by and large, a paper on integration and differentiation - as long as you're sound on that, it isn't too bad.
M1 was probably my favourite module. I had a "eureka" moment halfway through the course and suddenly everything made absolute sense. Which is always nice.
S1 was atrocious. Not that I couldn't do it, but it was mind-numbingly boring.
Also, depending on which exam board you do, try and suss out the way they ask questions. I did OCR MEI and sometimes the wording of questions made you want to rip up the paper..
In short - you should be fine. Just put the work in and make sure you don't get left behind in class and ask for help when you need it. At my school they ran an after school maths session every Wednesday and Thursday. I went along when I needed help with something (usually vectors) and it was a huge help.