The first thing you need to do is go on to https://github.com/crowdresearch/crowdsource-platform, and click on fork.
A fork is basically a copy of a repository. Forking a repository allows you to freely experiment with changes without affecting the original project. Once you are sure of your changes, you can raise a pull request to the original project to merge your changes to the original project.
When you have forked the project, you will find it in your github account