By joining the affiliate programs: Adsense, Ebay, Amazon and Kontera. Not all suit everyone and some people are refused. However - it's an individual process.
Do a search for the affiliates, in the search box.
Once you've joined Adsense for eg, you earn through advertisements placed upon your hubs - which internet surfers click on when looking for further info that you may have provided them with. It's similar to advertising on TV - targeted ads attract a specific audience which in turn generates revenue.