Ive finished my GCSE's but remember them well.
Here's my best advice:
1. GCSE's are really pretty easy. As long as you are reasonable intelligent and start revising a couple of months before the exams, you are 100% going to get all A's at least. I got 2 A*'s, 9 A's and a B(I got a B in GCSE PE and I did the entire GCSE in 4 weeks). All my revision was done the day before the exam for about 5 hours from 7 till 12. Thats it.
2. GCSE's are not important at all. All GCSE's are useful for is getting the required GCSE's for the A-levels you have to do. Most of your teachers are probably telling you that GCSE's are the most important exams you will ever do. They are lying just to scare you into working. As I said, I got all A's but having got all B's I would be in the same position. Uni's only look at your GCSE's if they have literally nothing else to look at and every top Uni has interviews which are always the defining factor (along with A-levels) that they decide from.
Seriously don't stress. You are in year 10 which gives you another year and a bit to get better. Just wait till A-levels hit you like a truck xDD.