As a beginner, I will not recommend you to contribute to an open source project on GitHub. Reason is you will get a lot of frustration and anger!
When I started GitHub interaction I learnt the hard way about contributing to GitHub as a beginner. I am mentioning some of them down-below:-
You won’t find any beginners-friendly open source project out there:-
This is the truth you will not find any real project out there that is meant for a beginner. Most of the projects are complete, you will be solving a particular problem and are designed to be robust, secure and flexible.
Most likely your patch won’t be accepted:-
Yes, it is also true that I know, the talk is about contributing to an open source projects on Github. No one will tell you how to do it.
Now I am mentioning how you should contribute to open source projects on GitHub:-
Firstly identify a simple problem which you could do by yourself:-
The problem that you will select should be something that not only you needs a solution, but a good for other people as well. When others are also interested then your project is most likely to be successful.
You problem does not require to be a complete application. Instead a do some small fix or automation that gives you a chance to learn GitHub as you also learn to code. Talking about the Git workflow this is something that takes some time and effort to get accustomed to.
From day 1 you should write the code and push to GitHub:-
Always remember that you are not just learning to code: you are learning GitHub and it is very important to know how to contribute to open source projects. Getting used to the workflow is key you must know how to create branches, knowing the master, push, pull, merge, rollback and stuff.
If you want to learn more about Github, then here is an exhaustive 3 hours long and informative tutorial on Github:
If you are more into reading, then you can have a look at the GitHub tutorial.
And, if you are interested to learn GitHub and want to master it with the industry-oriented case studies (which is necessary for every programmer) you can learn it by taking up the GitHub training course.