There's no better guide or place to start than just jumping in and making any contribution. My first contribution was adding a Contributors section to someones README.md on GitHub. My second contribution was adding a "Back to the top" button on someone's website. All contributions are good contributions.
Excuse me while I do a little self-promoting here but I do have an OSS project that I would really like to gain some contributors. Express Admin Area is a web GUI for database management. Using Express and React so front and back end devs are both more than welcome to make a contribution. If you're more comfortable on the frontend please.. PLEASE add some CSS to this project. There is also a few Issues that you can take a look at and feel free to open any even if it's just a question.
Now, you probably want bigger and more popular projects to contribute to so go ahead and give Code Triage a look over.
Awesome. Now, let me ask the even bigger newbie question... HOW do I contribute? I've never worked on a group coding project before.
How you specifically contribute is going to change per project. Most mature and popular OSS projects are going to have a Contributing.md file that would walk you through their process and what they expect from a contributor before attempting to make a PR. React Contributing.md.
For my project, Express Admin Area, you'll notice there is no Contributing.md and that's because I don't care. If someone does something blatantly wrong then sure I'll correct them or ask for a fix but it's pretty laid back.
Now if you're wondering about how to get your code into their code base then I would look over How to contribute but the idea is pretty much
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