"Open Advice is a knowledge collection from a wide variety of Free Software projects. It answers the question what 42 prominent contributors would have liked to know when they started so you can get a head-start no matter how and where you contribute. "
Good luck!
First, I made a habit of contributing to open-source everyday, it might be hard in some situations such as traveling, etc, but it helps you learn A LOT. You won't believe how much I've learned by contributing to open-source.
First, explore GitHub, using http://github.com/explore or by just browsing people's profiles, etc.
Subscribe to Changelog Nightly which sends you a daily digest of interesting github repositories. https://changelog.com/nightly/
Also look for other places to contribute, such as Mozilla, you can contribute to Mozilla products easily, they mentor you (the mentors are friendly and great) and you will learn a lot working on big projects (There are a lot of opportunities to work on using JavaScript, see whatcanidoformozilla.com)
As you go further, you'll even find more ways to contribute.
Don't forget that creating something and publishing it as an open-source software is a contributon and it's great! So if you find an idea which might help people, code it, open-source it, share it!
Good luck!