Just start contributing. There's no better way to learn than reading and understanding code. For me the most important item is a project that you personally connect with.

In my experience most projects are very kind to new contributors and someone will usually step up and gently guide. Quality code reviews have been most the effective mentorship I've had in Open Source and out.

Start helping users in the community by helping in forums, stack overflow for your chosen favorite Open source program. Write good tutorials in your blog and also starting contributing modules/plugins/addons anything which enhances the project and more you get involved, more you will learn and will be in better position to decide to excel to become top contributor to your favorite open source program. I applied the same what I wrote for my Fav WordPress program and now I love it and also have built a business around it.