Learning programming: so I’ve learned the basics of Python, what next?

Lots of people (particularly people associated with LessWrong) are telling me I should become a computer programmer; in response I’ve taught myself a little Python using this site, written a couple Python scripts on my own, and just now sent in an application to App Academy. But if I don’t end up going to App Academy, what’s the best way to develop some actually marketable programming skills? I’ve heard people recommending getting involved in open source projects on Git Hub, but when I looked at Git Hub I found it overwhelming, with no idea of how to find a suitable project to work on. Advice?