It depends what your blog is about, if you have a blog about dogs then your going to want to add products about dogs like dog food, dog toys, dog books etc... Your not going to add something like loosing weight etc...
Hello. There should be some specific connection between your blog and those products you choose to your google blog. So, in this case I want to agree with masterptc. It’s obvious. And you should keep it in your mind. It will help you to find useful products to your google blog.
Do you mean about setting up a blogspot blog? in that case the best thing to do is to generate traffic with the right keywords and using adsense.