Google Apps Script does not require setup or installation. The only requirement is a Google Account. A Gmail account works as well as a Google Apps for Work/Education/Government account. You can create a new Google account by going to accounts.google.com
Start your first script by going to script.google.com. You can also access Google Apps Script under the
tools -> Script editor... of many Google Apps i.e Docs, Sheets, Forms etc. Google Apps Script can also be added directly to your Google Drive with the
Connect more apps.. feature.
Official documentation can be found at developers.google.com/apps-script/.
For app-scripts to run they must contain a code.gs file. The code.gs file must contain a function named doGet (standalone scripts) or an onOpen function (addon scripts). The quick starts in the documentation contain examples.
If an api is turned on in the app-script it must also be turned on in the developers-console. However, the developers console contains api's that can be turned on but do not appear in the app-script interface. For example, Marketplace SDK needs to be turned on in the developers console before the app can be published to the Google play store or to a G suite domain wide deployment.
For Google apps for education/work/government there are settings in the domain admin console that can be adjusted to allow or disallow app-scripts to run.