[schme]

Find the kind of system you'd be interested in working on and start building one yourself. For me it was a game engine + an editor for it, but it could be anything you find the motivation to build.

Start with a book or a few texts/tutorials on the subject and begin building. Along the way you'll find the questions and choices involved. Find the answers from the internet or books. This is when I'd recommend some open source projects (not earlier) to see how they solved the specific problems. If you just go into an open-source project you won't have an understanding of the problem, just the answers, so it won't help you nearly as much.