I think you are looking for a "stock" answer but I don't think such an answer will serve you well. Many things sell well on the net and there are many ways of promoting them, but the question is whether you can promote and sell these things well.
You should start by looking at the products and services that, based on your unique talents, skills and interests, you are the most suitable to promote and sell. Then, among the promotional techniques, services and products, isolate those that will be most profitable for you in terms of time and money spent, and money earned.
And then you just might have a profitable Internet business. :)