Depending on what your knowledge is, what languages you enjoy or what is of interest for you you should just get in contact with people. Most of them are happy

Depending on what your knowledge is, what languages you enjoy or what is of interest for you you should just get in contact with people. Most of them are happy you want to contribute and will help you. Some resources: IRC-Channels, Google Groups, Github. I started working on documentation for a project because I wasn't really ready to work on the core (both because I didn't knew the project that well and because the whole git fork-pull request workflow was new to me). I learned a lot about contributing and now contribute to a few small projects.