The real answer to this is that there is no one size fits all type of answer. For instance some people have a product that they want to sell and they sell it on amazon, ebay, etc. Others will build their own website for selling that same product. Some people have no way of making a product or no idea for a product so they get into affiliate marketing and they can make good money off of that. Some make a blog and write and write and write about a specific subject and have adds on that page that give you an income.
Those are just the thoughts off the top of my head, I love listening to Pat Flynn on the smart passive income blog/podcast and he will give you every possible way to make money online.