Contributions to the Exercism code base are made via GitHub.
As indicated by its name, GitHub uses git as its version control system. If you're unfamiliar with Git, the best online resource is Pro Git.
In case you are not familiar with GitHub and the contribution workflow, here are some resources that might help you getting started:
You can read more about how to make sure your contribution is welcome in the How to make a great Pull Request guide.
We also have several Exercism-specific Pull Request guides:
There is also GitHub-specific tooling you can use: