If this is the first time running the development environment run yarn from the terminal to install the dependencies. If you do not have this command, see the docs for installation instructions. If you do not have node install, you should install it.
Next you are ready to run the development environment. Do so by running yarn start from the terminal. This will start up a development server which you can use to navigate to the application. (It'll tell you the URL but it'll probably be http://localhost:8080). Keep this process running and builds will be triggered whenever you make a change.