Hola, puedes ver la documentación oficial
https://firebase.google.com/docs/auth/android/password-auth?hl=es-419
mAuth.signInWithEmailAndPassword(email, password)
.addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
@Override
public void onComplete(@NonNull Task<AuthResult> task) {
Log.d(TAG, "signInWithEmail:onComplete:" + task.isSuccessful());
// If sign in fails, display a message to the user. If sign in succeeds
// the auth state listener will be notified and logic to handle the
// signed in user can be handled in the listener.
if (!task.isSuccessful()) {
Log.w(TAG, "signInWithEmail:failed", task.getException());
Toast.makeText(EmailPasswordActivity.this, R.string.auth_failed,
Toast.LENGTH_SHORT).show();
}
// ...
}
});