It will vary from person to person.For somebody they blog to sell products, somebody to discuss on various topics.

Every do it based on their personal choice and i do for sharing and gather knowledge(online marketing).

For a making a product selling blog, you need to make your page attractive, write a good article about the item you are trying to sell and place your affiliate link in the buying option.They will be redirected to the products homepage. I personally don't have any so cant give you the link.