Before anyone can post a review, we need something for them to review!
In this site, I'm going to use a post for each restaurant that people will be able to review. If you're already using posts for your blog, you might want to create a separate post type for the products or services you want people to review.
Note
If you want to create custom post types, you can do so using a plugin or by writing code in your theme. A plugin that will help you do this is the Custom Post Type UI plugin, available at https://wordpress.org/plugins/custom-post-type-ui/. For a guide to coding the post types yourself, visit http://premium.wpmudev.org/blog/creating-content-custom-post-types/.