Affiliate marketing, blogging, email marketing, creating/selling your own ebook, offering services/products on Fiverr etc. etc. This question is too general. There are a lot of ways to earn money online but it'll take time and dedication. Do your research -- find something you'll put some effort into and start working on it.
Try some affiliate programs like ClickBank. If you're good at marketing digital products, then this can work out well for you. But like icegin mentioned, there are a wide variety of things you can do to earn money online. Do some research and see what you are willing to put effort into.
I could try sites like clickbank, recently i registered paydotcom. but i have no idea where to find customers :/