Add a LICENSE file to the root directory of your repository. Github has several templates you can use:
- Go to the root directory of your repo on Github and click "Create new file".
- Name the new file "LICENSE". If done correctly, a button labelled "Choose a license template" should appear. Click on it.
- Read over the different license templates and pick the one you want to use and then click "Review and Submit".
- Commit the new LICENSE file to your repo.