The highest grade you can get in the first year is an A as it is AS. But for maths, go through the dr frost maths PowerPoints that has all of the chapters covered in the pure maths. It also has the mechanics and statistics section covered as well. Then watch some YouTube videos such as Zeeshan and haberdashers, they are really helpful. Have a flick through the notes in the textbook that states the important points that is needed for the topics in the chapter. Look at the examples as well, then do all of the textbook questions including the mixed exercise. If you want you can push yourself and do the challenge questions. You also also consider doing questions from elsewhere after you completed with the textbook for extra practice. If you or your school has access to Pearson active learn then they should have the textbook within it. CGP isn't good for Maths btw so I recommend you don't use that. The questions from that are basic. So preferably use the pearson textbook.
For bio and Physics, the CGP revision guides are definitely the best ones out there. It would be best if you use it a very effective way. This is when you make notes and flashcards after you read it and understand what's going on. When you know the content pretty well, you should do a lot of exam style questions from online resources like Physics and Maths tutor after that. Just doing some questions from the book aren't enough, they are just a trailer. You need to ensure your getting enough practice. Free science lessons is a good channel on YouTube for biology. But don't just rely on the CGP revision guides for science, they do miss some things out which the spec wants. I would say when revising, use the specification from the exam board to guide you on what you need to know and if its included in the revision guide then just use that. If it is not, then get the notes from PMT, mmerevise, revision science or the study rocket. Seneca learning and quizlet is also a good tool to test yourself and retain memory. So for Physics I would suggest you use that.
If you don't get Physics that much, then yes definitely get a tutor. But ideally, you should balance out all your subjects equally and spend a lot of time in them. My advise is to create a timetable and focus on 2 subjects on a day.