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You will most likely start with Core Science and start with Biology B1 or Physics P1. Once you find out, make sure you have the correct revision guide (which will either be provided by your school or you might have to buy yourself from W H Smith or any book store). This is the most effective way of memorizing all the information, after you learn one topic in one lesson, once you're home from school then run through it again in your revision book and rehearsing the notes you made in class. Past paper questions are miracles! Find past papers online from the Edexcel website under GCSE from 2011 and then print out the questions you can find for the relevant topic. Don't print out whole past papers, only the questions for what you're revising (although you can for future use as you will need all the questions eventually). As always with any exam, don't try to cram all the information into hours every single day. Take breaks and relax! You can't think and learn properly if you haven't got time to chill. Have about an hour or two session daily or every other day with plenty of water and snacks. You can find the specification PDF in the Things You'll Need section of this article, this is a PDF from the exam board clarifying exactly everything you need to know to ace the exam. After learning each topic from the revision guide and doing some past papers, highlight if you think you've aced it with a green highlighter, and if you're still unsure highlight in orange or pink as a warning to run through it once more. Go back through your orange highlighted parts of the specification and revise from the start again, you don't need to revise every single topic again but it's good to revise twice because you probably won't exactly remember the first topic which you revised long ago. It's to refresh your memory, the more you rehearse something, the better get. Ask your teacher to give you full past papers or mock papers, it's time to put all that knowledge to test and practise for the real things. Ask someone to mark it using the mark scheme and then see where you failed, and which 6 markers you struggled on. Go back and then try to revise the topics you struggled on further. Relax, if you worked hard and didn't quit on revision you will have gotten at least a B. Don't revise last day or worse not revise at all, you will find that you can't get better than a D. Always revise!
Find out which subject and unit you're doing first. Use your revision book at home after each science lesson. Use questions from past papers. During revision, take breaks. Print out the specification for the unit. Once you've gone through every topic, it's time to refresh. Practise with full past papers. On the exam day.