Michael Willian once you deploy your application to heroku using “git push heroku master” you will be able to see the url to access your application. Also you can login to heroku website and navigate to https://dashboard.heroku.com/apps to see your application and there will be Open app button at the top to access your application.
If you are running the application locally you can access it using either localhost:3000 or 127.0.0.1:3000. Let me know if that helps