New to GitHub? No problem! You're in the right place!
Fork this repo by clicking on the Fork button on the top right of this page.
After that, you'll be working off of your Fork of this repository, which is effectively a snapshop in time. Here, you will have a copy of this repo and you can modify it to your heart's content. Once done, you can do a Pull Request to propose changes to this main repo so everyone else can benefit from your contributions. With this repo, it's purely for practice, so don't worry about solving the world's problems on this repo.
As time goes on, this repository will evolve and your Fork will be left behind if you don't keep it updated. Remember, it is a snapshot in time. Be sure to Fetch Upstream prior contributing more so you have the most up to date copy of the repository before you starting adding to it!
Above is an example of Fetch Upstream combined with doing a Pull Request, which is what you should do when you have something new to the repo you'd like to add to the main repo. Ideally, Fetch Upstream -> make your changes -> do a PR to the main repo. Then, you wait for the Owner/Collaborator to approve/deny/request changes to your PR.