Alright, first things first. Head over to the Firebase Console, pick your project, and then navigate to the ‘Authentication’ section. Click on ‘Getting started’. In the Sign-in method tab, you’ll see a bunch of sign-in providers. Just toggle the enable slider for the ones you need.

If you’re looking to set up email and password authentication, here’s what you do:

• Go to the Firebase console
• On the left side, click Authentication
• Click on the Sign-in method tab
• Find Email/Password provider, flip the Enable switch, and hit Save

Next up, you need to set up a service account and generate a new private key for your Firebase project. This key is crucial because your application server will use it to create custom authentication tokens.

• Go to Firebase Console
• Click on Project Settings
• Click on the Service Accounts tab
• Click on Generate New Private Key

The key file you download contains your project's credentials. Keep it safe! It has full access to all your Firebase project resources.

Now, you need to mint a custom token on your application server using the Firebase Admin SDK. This token will be given to your clients so they can interact with Firebase.

Here’s a Node.js example to generate a custom token:

const admin = require('firebase-admin');

admin.initializeApp({
  credential: admin.credential.cert({
    projectId: '<PROJECT_ID>',
    clientEmail: 'foo@<PROJECT_ID>.iam.gserviceaccount.com',
    privateKey: '-----BEGIN PRIVATE KEY-----\n<KEY>\n-----END PRIVATE KEY-----\n'
  }),
  databaseURL: 'https://<DATABASE_NAME>.firebaseio.com'
});

admin.auth().createCustomToken('<UID>')
  .then((customToken) => {
    // Send token back to client
  })
  .catch((error) => {
    console.log('Error creating custom token:', error);
  });

Make sure to replace <PROJECT_ID>, <DATABASE_NAME>, <UID>, and <KEY> with your specific project details.

Once your client gets the token, they can use it to authenticate with Firebase services. Here’s how you can sign in the client using this token with the Firebase SDK:

firebase.auth().signInWithCustomToken(customToken).catch((error) => {
  // Handle Errors here.
  var errorCode = error.code;
  var errorMessage = error.message;
  // ...
});

To control who can access your Firebase Realtime Database, you’ll need to set up some security rules. These rules determine who can read and write data, validate the structure and size of the data, and more.

Go to the Database section in the Firebase console. Make sure you select "Realtime Database" and not "Cloud Firestore". Then click on the Rules tab. Set the read and write rules according to your needs.

Here’s an example rule that allows authenticated users to read and write:

{
  "rules": {
    ".read": "auth != null",
    ".write": "auth != null"
  }
}

With these rules, only authenticated users can read and write data in your Firebase Realtime Database. Don’t forget to click "Publish" after you make changes to the rules.