To learn the basics online, you’re actually going to have to know some basics, so that you know what to search for (eg World War 2). By trying to do it yourself without taking a class, you will have to put a lot more work in and you may find it quite daunting to work out which parts of world history to focus on. (At least in a class the tutors have worked all that out for you and can structure your learning better.)
For a more general overview, perhaps you could do searches on the relationships between different countries and look at the history of those – you would learn a lot about the world by examining how different countries have related to each other over the decades/centuries.