Welcome; If you'd like to contribute, firstly, thanks!
The quickest way to get the source without contributing is;
git clone https://github.com/Skenvy/Sudoku.git
If you want to contribute to the source, you'll need to fork this repository, clone it, and create a development branch. Contributions should be received as pull requests from your development branch on your fork.
You may work on any existing issue, by forking this repository, creating a development branch, then adding a pull request.
Work on bugs is more likely to be accepted quickly.
Any fix or new feature should include an appropriate set of testing, which may require novel approaches to be considered.