To edit and run Javascript code in the browser we will use a service called JsFiddle. JsFiddle is a service to exchange and test out short snippets of code in the browser, which makes it ideal for making small games. For this exercise I prepared a fiddle where the Phaser Game Engine is already loaded. You can just start coding without an account but to save progress you will need one. In the toolbar at the top you can see the buttons to run your code and to save (requires an account).
Then you can go through to the next step.