Took GCSE Japanese last summer, mostly self-studied, and got an A* - if you can sit it under the old spec, please do so, as you can use an electronic dictionary during the exam and the examiner reports basically tell you how to answer questions!
So, top tips:
- learn (to write) kanji via learning radicals. There are sites that will help you with this - I use wanikani (not free, but I had a 50% off code and am now in my second (and last) year of using it). This allows you to break kanji down and can be especially helpful with pronunciation, as you'll notice that kanji with certain radicals are almost always pronounced the same way.
- around 2 months before the exam, get a tutor and just make them mark as many papers as you can do. 3/4s of a GCSE is just knowing how to answer the exam paper with good style. Until then, use lang-8 to have native speakers correct your grammar - practice this as soon as you know enough to form basic sentences, and make sure you learn all of the GCSE grammar + some impressive "set phrases" that are a little above GCSE, e.g. と言われています (it is said that). A good free grammar guide is Tae Kim's Guide to Japanese Grammar - should have all the GCSE stuff.
- try and sound as "Japanese" as possible when you speak! It's small things like using Japanese fillers instead of English ones (etou/ano instead of um), and practising your accent until you can sound similar to a native - youtube is really useful for this.
- Whilst the GCSE Japanese listening is possibly the easiest of the exams, don't entirely neglect it either. Listen to anime or readings of Japanese kids books on youtube, or mock exams for JLPT N5 (in fact, learning the vocab for N5/N4 is what I did for GCSE and there wasn't a single word I couldn't understand on any of the papers).
Most importantly of all - stay passionate and hardworking. Japanese is one of those languages where you start by breaking through huge boundaries and then stagnate for a while - but don't get discouraged and keep steadily working away. You'll get there!