This is basically my scenario. Got the BS in CS by rushing my degree, but getting into FOSS is proving harder than I would expect, mainly because every project has "some" technology, from automake to some framework like qt or something else that I never learned and thus feel helpless trying to get into, especially on anything with more than 200k LOCs. Getting into something like that is getting through a brick wall with a twig.
The tech field is like that. Work won't be any easier. In fact, expect major libraries and build tools that are internal to a large organization to be less elegant and documented. Learning stuff like that quickly is a skill you're going to need.
I don't have a lot of advice besides "practice".
Well, I do have some. Don't be afraid to open up the tool's source and read it. Try to design the tool yourself and you'll get a better sense of how it's likely to do things.
Actually the trick is to choose the project that uses technology you already know. If you know Java better than C++, contribute to Java projects. At least at first, do not choose FOSS projects to contribute based on how impressive it would look.
