Github can be a little tricky to those that aren't familiar with it - this guide will walk through each step of forking, cloning, and submitting a pull request with QMK.
If you're apart of an organization, you'll need to choose which account to fork it to. In most circumstances, you'll want to fork it to your personal account. Once your fork is completed (sometimes this takes a little while), click the "Clone or Download" button:
You now have your QMK fork on your local machine, and you can add your keymap, compile it and flash it to your board. Once you're happy with your changes, you can add, commit, and push them to your fork like this:
Your changes now exist on your fork on Github - if you go back there (https://github.com/<whoeveryouare>/qmk_firmware), you can create a "New Pull Request" by clicking this button: