Open the .env file and write your token in the appropriate location after the =
Note that env files are not json or js files, do not put quotes or space anywhere. It should look like:
DISCORD_TOKEN=MTg-this-IzNzU3OTA5NjA-is.not-DCeFB-a.real-r4DQlO-t0ken-qerT0
Save the file, and start your app, and it should login.
For more setup instructions and a guide to write your own bots, please visit An Idiot's Guide