Do you want to be a maintainer or a contributor?
I recommend to pick a project you are already using and you like. Send bug reports and look how/where they fix them. If you know how to fix it, try to send a pull request.
If you have a feature you like, try sending a pull requests. Start with small improvements, like 1 or 2 days of work. You never know if your change is in the vision of the maintainer, or it breaks something or the maintainer is just a moron. Be prepare to fix your PR because each project has it's own secret weird rules. Spaces or tabs? Did you add tests for the fix/feature?