You can follow the below-mentioned ways to contribute to the GitHub open-source projects:
Firstly, you will need to go through the documentation to understand what the project does and what it does not basically, know all about the project.
After that, you need to do the fork and clone the repository and create a development environment on your local server by looking at the Readme/contributing/setup file which is given in the project itself.
Now comes the most important part where you use certain features in a demo app to understand the API in a much better way. As it will help you to understand the kind of problems a developer can face if he is using this project and you will be able to understand how you can make an API faster.
Then you need to go to the issues tab in the GitHub repository and try to find out if there is something where you can help others.
You have to always write test cases for the thing that you have worked on.
At last, you need to create a pull request on the main repository.
Through my observation, I believe you are good with C++ programming language, apart from all the factors that I have shared above, you can check for the open-source projects about C++ on Github, if you have any suggestions or you can make the project better by including new features using your C++ skills, then create a fork on that repository and commit the new changes or features that you have added, become a contributor and keep your suggestions open for other coders also. Such contributions bring good limelight over you, which you can actually show off.
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.