Subscribe to the mailing lists of one of these projects and try to build the project from source. Check the bug trackers for things that are not very interesting but need fixing anyway. Then prepare a patch and try to upstream it (the method depends on the project).
As you work on the little things you will slowly gain a better understanding of how the code base is organised and how you could contribute more significant changes.
Valuable contributions outside of writing code differ across projects, but most lack current or understandable documentation. As you learn about the project and read whatever documentation might already be there you will certainly find things that can be clarified.
As you work on the little things you will slowly gain a better understanding of how the code base is organised and how you could contribute more significant changes.
Valuable contributions outside of writing code differ across projects, but most lack current or understandable documentation. As you learn about the project and read whatever documentation might already be there you will certainly find things that can be clarified.