That's a BIG question and there are many answers. Online or Offline, the most basic answer is to find a need and fill that need.
Online most people make the most money doing one of the following...
- selling other people's products (that's Affiliate Marketing)
- building Network Marketing businesses
- selling their own products/services
- helping "brick and mortar" businesses sell their products online
- coaching (for a fee) other people to do any of the 1st 5
There are many other answers to your question but it really starts with you and what you are interested in to the point of being passionate about and what you are trying to accomplish.