Hey Samuel, you've come to the right place!
A C- average is actually pretty damn good for English, so don't be discouraged! Typically that means you've got a handle on the basics, and you just need to refine your knowledge to get up to that B/A territory. The most useful thing you can do know is work out what sorts of things are holding you back. Chatting to your teacher about your essays or any areas you're struggling with is a good place to start.
As for 1984 (let's set aside the exam conditions for now): if you feel like you don't have a good grip on the story (i.e. what happens in the plot, who the characters are, and why they do things) then I'd recommend looking up some summary resources online. You'll find a
tonne of stuff for 1984, so just schedule some chill study time to watch a bunch of YouTube videos or read some crash course guides. (You can also check out the ATAR Notes book if you prefer a physical resource; you can view a preview of the first few pages, including a condensed summary
here).
From there, you can start working on 'the concepts' and what the themes and messages of the book are. This is also something analytical resources can help with, but a good starting point would be making a list of as many key concepts as you can think of (e.g. surveillance, loyalty, freedom, etc.) and then ask yourself 'what is Orwell saying about _____?' For instance, what is Orwell saying about loyalty? Well, based on how the Party corrupts Winston's loyalty towards Julia, perhaps Orwell is suggesting that loyalty cannot prevail over the need for self-preservation. This then builds up your understanding of the overall novel, making it much easier to eventually write essays that have strong arguments.
As for the exam conditions, don't stress about that for now! As you've said, you have plenty of time to prepare, so focus on delving into the text now and do as much reading/research as you can. Don't expect yourself to be able to handle an exam
now before you've spent enough time on the content - that'd be like just starting a new topic in Maths or Science and then being given a test on everything the very next day!
Typically the scariest thing about 'exam conditions' in English is not knowing what the prompt will be (and how the hell do you prepare for something when you don't know what the question is??) so essentially you want to start getting a broad understanding of the text asap. Again, coming up with a really big list of themes is super beneficial, because it means you'll probably cover most if not all possible key words that will be on your exam. Once you've got that list of, let's say, 20 themes, you can start grouping them together (e.g. putting 'surveillance' 'privacy' 'independence' and 'government oversight' in the same category) and write a practice essay about those ideas. Eventually you'll have talked about so many different ideas that you'll have the skillset to handle anything for your assessment task, even if it's not one of the exact words you've covered.
I hope that helps; best of luck with 1984! It's an amazing book to analyse so I hope you have fun with it!!