Hey there
I got 3 A*s for the IGCSE triple science award, when I was predicted Bs a month before! I became the queen of revision and this is how:
1) Find the specification for each exam. This will list everything you need to know.
2) Now buy the CGP revision guide for your exam and summarise each spec point using notes from the CGP revision guide. If you don't understand a concept, go onto youtube and watch a video on it (KhanAcademy is pretty awesome).
3) Once you've made notes on everything, read through your notes once or twice and then do a timed past paper. Try not to do it before you've made notes because there are a limited number of relevant past papers (they have different specs). Start with the most recent as the questions are of a similar type to what you're likely to get in your exam.
4) Then go over your past paper and look at what you'e got wrong. If you got it wrong because of a stupid mistake, make a note of it in your head. If you got it wrong for any other reason, firstly, look through your notes. If you can't see your error, look through the CGP revision guide. If you still can't see it, look at your textbook. And if it's not there, go and talk to a subject teacher about it. They love it when kids ask for help too, so don't be shy about it. Then take notes on your corrections.
5) Keep doing past papers with the technique that I just mentioned.
This is literally a fool-proof way to get near to full-marks in your exams.
Hope it helps you and good luck in June