Look for a project that holds your interest and that has bugs which seem within your reach to fix or it has missing functionality which you would use (and be willing to implement). If you're not looking for a coding role try to lend a hand with what skills you currently are trying to build.
One project that I'm currently maintaining first caught my interest when I tried to connect a MIDI piano up to a Linux system and I found several of the open source options had issues compiling. At that point I submitted fixes to get it working to my satisfaction and I have continued to help improve upon it since then.