As a C++ developer and boost user, I advise against starting to code for it. API design and implementation are quite hard as opposed to "normal" programming, as you need to factor several other problems: building a easy-to-use generic interface, mandatory in a library, is much more difficult to code than in project that works with specific data models. Also, I had specific problems with boost::filesystem API so I can tell you getting a bug acknowledged and fixed in a particular version might be frightening at first(no, I don't want to update my boost version to get it... I want it backported).
I suggest you find a piece of code that you are interested and find useful and follow this simple roadmap:
1.Install it and use it
2.Subscribe to user/devel mailing lists
3.Write missing documentation and unit tests for components
4.Offer to implement features/fix bugs that have been appearing for some time but have a low priority.