Decide where you want to host it. Do you want to post reviews on your blog? I recommend Blogger as Wordpress.com gets stingy about affiliate links. Do you want to pay for hosting you blog? then I suggest buying a domain name and webhosting and going the route of self-hosted Wordpress. Fully-customizable and no worries about affiliate links then.

There's a lot of decisions you got to make with your blog before you start it. But start free! It helps. Once you know you want to invest a lot of time / energy, then move up to paying for your blog.