So should I start testing a game now to prepare? And what will the review be graded on?
It's easiest to pick a game you really liked but not one that's super popular. You cannot pick an MMO.
Review isn't graded per se, but I look at the content of what you write. Grammar doesn't have to be perfect, but if it's really sloppy, hard to read, etc., you're going to have to make some changes.