1.
Referencing - make sure that you are referencing correctly, and consistently. Use the same system for everything that you reference. The Harvard system is a good one - it is easy to learn and widely accepted
2.
Formal writing - an easy way to begin writing more formally straight away is to expose yourself to formal texts. Start reading well-written books - non-fiction books are written in the style you need to learn, but there are plenty of eloquently-written fiction books around that will also help you with a slightly more formal style. I've always found Jane Austen to be a great one to read because it's a good story but also fantastic writing, and close to the style you're looking for.
3.
Evidencing - in order to provide evidence to support a claim you're making, you need to completely understand the context around which you are making your claim. If you fully understand not just the subject matter, but also the wider area of the subject, then you can make a claim that is much easier to evidence because you know exactly what you are looking for when finding such evidence.
4.
Take your time! Don't feel like you have to get it all written in a couple of days. I'm assuming you've already done the bulk of your research, so now you've got to the writing part, you need to section it out. Remember, you've got school and extra curriculars to maintain whilst you're doing this, and it's vital that you don't prioritise the wrong things at the wrong times. Set yourself a goal, like 100 words a day, or 500 words a week. It doesn't really matter - just make it achievable, but also motivating.
5.
Final thing is fairly obvious - get people to read your work. Physics and maths teachers are obvious choices - they know the subject matter, and will be able to guide you on your understanding. But also ask an English teacher to review it - they can guide you on your writing style, and suggest other ways of phrasing ideas to communicate them more clearly or more formally.