I know the feeling! I started working in a research team straight out of my bachelor's and was also intimidated by the first review invitation.

In my (in)experience, the most useful skill to have is being able to read other papers since no one would be familiar with everything else. I think specific instructions like this are helpful for us newer reviewers: https://2020.emnlp.org/blog/2020-05-17-write-good-reviews

Also, one thing that really helped was the conference we submitted to (ACL I think) had a program where first time reviewers would have their review looked over by a senior reviewer before submitting. If this doesn't exist for your venue, maybe do this informally with a more experienced colleague.