Start a Blog: Well I am a blogger in midst of many experienced blogger friends on the internet (hello there :), if you are one of them), so I will not be commenting on how you should write and what could be the strategy to choose a topic. That is your puzzle to solve, but I know many famous bloggers out there who are earning pretty good.
Build a Mobile App: If you are into computers/technology and have interest in coding languages, learning a programming language like Java and Python which is used to create mobile applications could be a very good option. Many applications like WhatsApp, Angry birds and many more earned and still earning a fortune for the developers who just created a simple application while sitting home.
Once you start your online business venture you already reach the halfway and the rest of the way could only be achieved if you KEEP GOING. Stay Awesome!