I strongly recommend trying to contribute to a project that you personally use. Once you've been using it for awhile, you'll be better equipped to make changes to it + you'll maybe find bugs that you can fix. While you're doing this, mark yourself as a watcher of the project on Github, that way you can see bug reports come in and maybe fix them.

You can try to contribute to just any project someone here suggests, but I find it hard to get motivated + be skilled enough in that language/project to just start contributing. Can you talk about your current skills and what kind of languages/libraries you're using now, or are interested in learning?