There are many open source projects that need help. They are looking for coders, but they are also looking for testers, documentation writers, and translators. Any of these tasks will help you build your brand. Being a good member of the community is more important than being a genius coder.
The best way to start working with an open source project is to become active in its community. Find a project you are interested in and join the users and developers mailing lists. Watch the list discussions while you figure out how to build the project and get familiar with the code. When you are ready, let people know that you want to help. In most projects the right way to do this is the developer’s mailing list. Most projects have a list of bug fixes, enhancements, and other tasks they need completed. Pick an item from the list that suits you and give it a try. Repeat the process a few times. If that works out well and you are feeling ambitious you can start your own open source project.