When I find some interesting project (e.g. on GitHub) I often would like to use it and try it out to see how it works, but if there isn't a good documentation or some kind of tutorial it's hard to figure out how to use it.

So my question is: how do you approach such a situation? How do you figure out which classes are important and how to chain them to put them to use? What would you look at first?

An advice I found is to look at the tests (if there are any). But if there are unit tests for every class, how do you know which ones to look at first?