You're welcome to submit an older review for your first, but heads up that it might not gain any additional traction, since publication homepages are sorted by date (ie, it won't jump to the top of the page, but will get sandwiched in between stories of the same date). New posts tend to do better for this reason.

There are a few "what to review" articles to get you started under the "Become a Reviewer" tab on the homepage that can give you some ideas. Although the Monthly Theme is currently on Hiatus, you can also look through the previous monthly themes for writing prompts as well. :)