As important as reviews are to an author's building of a fan base as well as generating sales, writing reviews for other's is also important.
When you write a review for another writer's book, you get your name out there, and that's very important. Anytime your name or your novel shows up on the web, it's an opportunity for a curious individual (or several curious individuals) to click on it, ultimately leading to a possible sale.
Another positive to reviewing books is the connections you make with that author. They may quite possibly appreciate your review and connect to you through their goodreads account or facebook or your blog (or all of the above and more!).
Think of yourself as building a web, every strand connecting to make another strand functional. And as much as I hate - fear! - spiders, webs are beautiful, fascinating works of art.
A good place to learn about the art of writing good reviews is any one (or more) of the multitude of bloger review sites. Here are a few to look into that I like, but there are thousands, covering every type of writing. http://novel-moments.blogspot.com/, http://livetoread-krystal.blogspot.com/, http://vidya-booksaremagic.blogspot.in/, http://reviewsbymolly.com/
So begin today. Start with the last book you read and write a review for it in one or more locations (ex: goodreads, smashwords, amazon, so on...). And remember to do unto others as you'd have done unto you, or what goes around comes around. You get the idea. ;)