When you are just starting with Firestore, you should read and follow the instructions in the Get to know Firebase for Flutter codelab, specifically steps 3 and 4. These instructions include some one-time setup actions, such as installing the Firebase CLI.
Once you have successfully configured a Flutter app to work with Firestore, you might find it a bit faster to refer to this page, which detail the minimal steps to create a database with Email authentication. There are just three steps:
Go to the Firebase Console.
Click the “+” button to add a new project.
Enable (or not) Google Analytics.
In the Project Overview Pane, expand the Build menu, and click Authentication > Get Started > Sign-in method > Email/Password > Enable > Save.
Also in the Build menu, click Cloud Firestore > Create database.
Select Start in Test mode.
Click Next and select the database location.
In the top-level directory of your project, run:
flutter pub add firebase_core firebase_auth cloud_firestore firebase_ui_auth
Also in the top-level directory, run:
flutterfire configure
This runs a configuration script that ultimately produces a firebase_options.dart file in your project.
Note that API keys for Firebase are different from typical API keys and thus the firebase_options.dart file can be committed to GitHub.