I would say to start by reviewing products that you use around your home. This will give others a sense of your writing style. Make sure you include as much detail as possible as well as personal photos and/or videos in your review posts. When you feel confident about your review skills, contact blog-friendly companies and introduce yourself and your review services.
You can also apply to various group review sites like Mom Central, Mom Select, One2One Network, Team Mom, and Family Review Network ;-)
Most companies will be more than happy to offer an extra product to give away if they feel they will benefit from it (links back to their site, twitter followers, facebook fans, etc.)