Watch youtube videos on the topics you're weakest by checking your specifications at then do exam questions (using Physics and Maths Tutor). Mark them so you can see what key words you need to use to get the marks. Watch more videos if you're still struggling/ask your teacher for help. Make flashcards on the questions/topics you're still stuck on and use spaced repetition to help it make it stick. When you get more confident, do whole past papers (eventually doing them under timed conditions) to mimic the actual exam.
Make sure you apply your science knowledge & programming as that's the only way you can improve. Coding some simple programs allows you to problem solve which is super important.
Resources: Maths - TL Maths, Corbettmaths (plus his 5 a day) MathsGenie for lots of questions Science - Malmesbury Education (required practicals) Cognito, Freesciencelessons, Primrose Kitten CS - computingmrh, https://www.ihaz.codes/UoB/ Other - SaveMyExams, BBC Bitesize
All these subjects are practice so if you really work at it you'll be fine