The best thing to do is to learn a SEO, and become SEO Expert. Once you become SEO expert you can make your own marketing campaigns, sell your own stuff, affiliate or even open your own SEO Service and provide it to other people. You can set up various business with that knowledge, it's up on you what will you decide!
I think the best online business is advertising program like google adsense..Just create websites in profitable niches than monetize them with google adsense.. Well..It's just my opinion..something i believe..It might be wrong..