OpenShift doesn't host a Git repository for you. It relies on you having a separate hosted Git repository to pull application source code from. Alternatively, you can use a binary input build and push source code from your local system.
If you already have a deployed application how did you create it?
If it was a sample S2I application, you need to go to GitHub where the source code for that sample application was and clone it into your GitHub account. You can then check it out from your fork in your own GitHub account to your local system. You will though need to update the build configuration for the application in OpenShift to then use your fork of the original sample application.