In order to master your German GCSE exams and to achieve good grades you should ensure that you are familiar with the exact exam expectations. It will be beneficial for you to double check this with your teacher and ask for specific information regarding the syllabus, exam board and the best way to prepare for the exam. This information will be very relevant to you.
Please also remember that there are very useful websites you can use; for instance www.edexcel.com which is written by qualification experts and highly respected authors. You will also find some useful exercises and grammar tips on the following website: http://www.bbc.co.uk/schools/gcsebitesize/german/.
Another good resource for German grammar is www.mein-deutschbuch.de. Try and practise your spoken German as much as possible; ideally with a native German speaker and read some German articles and books on topics your are interested in and which excite you.
By learning more about the German culture and people you will have more fun and you will become more and more motivated in learning German.