First things first, you need to enable Firebase Authentication in your Firebase console. Head over to the Firebase console, pick your project, and then click on Authentication in the left-hand menu. After that, go to the Sign-in method tab and turn on the providers you want to use, like Google, Facebook, or Twitter.

To get Firebase working in your app, you need to add the Firebase SDK to your project files. Just include the Firebase core and Firebase Auth modules in your build.gradle (app level):

dependencies {
    //...
    implementation 'com.google.firebase:firebase-core:20.0.1'
    implementation 'com.google.firebase:firebase-auth:21.0.1'
}

Depending on which provider(s) you want to use, you'll need to follow specific setup instructions from Firebase.

For Google Sign-in, check out: Google Sign-In with Firebase Authentication

For Facebook Login, take a look at: Facebook Login with Firebase Authentication

For Twitter Log-in, see: Twitter Log-In with Firebase Authentication

You'll need these guides to get your app credentials and set up the sign-in provider's SDK.

Now, let's add the sign-in functionality to your app. Create a sign-in user interface in your app. When the user clicks the sign-in button, the app will trigger the Firebase sign-in intent, starting the sign-in process.

Here's a quick example using Google Sign-In:

private void signIn() {
    Intent signInIntent = mGoogleSignInClient.getSignInIntent();
    startActivityForResult(signInIntent, RC_SIGN_IN);
}

Depending on the sign-in provider you're using, handle the sign-in result. Here's an example of handling the sign-in process with Google Sign-In:

@Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);

    // Result returned from launching the Intent from GoogleSignInApi.getSignInIntent();
    if (requestCode == RC_SIGN_IN) {
        Task<GoogleSignInAccount> task = GoogleSignIn.getSignedInAccountFromIntent(data);
        try {
            // Google Sign-In was successful, authenticate with Firebase
            GoogleSignInAccount account = task.getResult(ApiException.class);
            firebaseAuthWithGoogle(account);
        } catch (ApiException e) {
            // Google Sign-In failed, update UI appropriately
            Log.w(TAG, "Google sign-in failed", e);
            // [START_EXCLUDE]
            updateUI(null);
            // [END_EXCLUDE]
        }
    }
}

Once a user successfully signs in, get an ID token from the provider and use it to authenticate with Firebase.

Here's an example for Google provider:

private void firebaseAuthWithGoogle(GoogleSignInAccount acct) {
    Log.d(TAG, "firebaseAuthWithGoogle:" + acct.getId());

    AuthCredential credential = GoogleAuthProvider.getCredential(acct.getIdToken(), null);
    firebaseAuth.signInWithCredential(credential)
        .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
            @Override
            public void onComplete(@NonNull Task<AuthResult> task) {
                if (task.isSuccessful()) {
                    // Successful sign-in.
                } else {
                    // Failed sign-in.
                }

                // ...
            }
        });
}