Being a book reviewer likely sounds like a plum job for many writers, who generally love reading as much as writing. Despite this, it's certainly not a pipe dream. There is indeed paid work available for book reviewers.
The first step is to obtain books on your own (at your own expense) and publish reviews on open platforms like Goodreads or Amazon.
This helps the writer in several ways. First, it keeps you on top of the recent releases in your interest areas and genres. This is important because reviewing focuses on recent releases (with a few exceptions). It also teaches you the process of writing a book review. Interacting with other writers, reviewers and readers will help to shape your prose. You'll also get the chance to observe their review styles. Last, you may begin to develop a following of fans who appreciate your reviews and writing style. It is from this following that you build your audience for later endeavors.