Okay, I don't know what year your in but I was in that very position last year. I had no hope of getting a C, I was completely rubbish at maths and to be honest, I only scraped a C and that was during my last maths exam of year 11.
Foundation maths is harder to get a C in because you need more marks, over 80% I think which means you really can't afford to drop marks on silly mistakes. Whereas, on higher you can afford to drop those marks because not only are the questions worth more 6/8 marks, but you need less than half of the questions on the paper to get a C.
If you're in year 11 and want to go to sixth form, please try desperately to get a C now as it will be so much easier, but you're school should offer retakes in year 12. Getting a C in maths is so crucial if you want to go into higher education.
What I did:
Had a tutor- I know these can be expensive, but some things are worth it.
PRACTICE PAPPPERRS- goodness me, I'm not saying they massively helped but they really got me familiar on exam questions and what to do.
And if you're in year 9- don't stress! You have two years left to get a C or even higher!