Congratulations!
From my limited experience I would say that practicing is really important.
I found from my video interview for another firm that a lot of the answers to the questions were things I had already written about in the application form.
Knowing why you want to apply to that firm and what makes you a good fit, as well as the skills you have and your motivations for a career in law.
Also pay careful attention to the email invitation as it may have some clues about what you will be asked.
I made a slideshow with some practice questions and created a 1 minute timer on it so that I could practice answering questions in one minute looking into my webcam. It's hard at first but gets easier.
Good luck!