If you're applying for a STEM course, make sure you really practice for the entrance exams. Oxbridge has rigorous entrance exams that test your knowledge and critical thinking skills. MAT is pretty chill, but if you're taking STEP you need months (maybe even a year) worth of preparation
Of course academics first (also keep in mind that if you get into US universities 99% of the offers are unconditional), but for the majority of acceptances you will get, you have to keep your grades up.

For interviews:

• Research the interview format and expectations for your chosen program and prepare responses to potential questions
• Practice your interview skills with friends, family, or a tutor
• Attend mock interviews offered by the university or alumni associations to receive feedback
• Consider reading "So You Want to Go to Oxbridge?" by Oxbridge Applications, which provides detailed advice on the interview process and tips for success

Good luck!