usernameTaken:
    [ID generated by push()]:
        Username: username
  users:
    UidFromRegistration:
       Email: emailFromRegistration
       Username: username

private void checkUser() {
    String username = mUsernameSingUp.getText().toString().trim();

    mDatabase.child("takenUsernames").orderByChild("Username").equalTo(username).addListenerForSingleValueEvent(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot dataSnapshot) {
            if (!dataSnapshot.hasChildren()) {

                registerUser();

            } else {

                Toast.makeText(SignupActivity.this, "Username already taken", Toast.LENGTH_LONG).show();

            }
        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });


}

private void registerUser() {

    //getting email, username and password from edit texts

    String username = mUsernameSingUp.getText().toString().trim();
    String email = mEmailSingUp.getText().toString().trim();
    String password = mPasswordSingUp.getText().toString().trim();

    //checking if email, passwords and username are empty
    if (TextUtils.isEmpty(username)) {
        Toast.makeText(SignupActivity.this, "Please enter username", Toast.LENGTH_LONG).show();
        return;
    }

    if (TextUtils.isEmpty(email)) {
        Toast.makeText(SignupActivity.this, "Please enter email", Toast.LENGTH_LONG).show();
        return;
    }

    if (TextUtils.isEmpty(password)) {
        Toast.makeText(SignupActivity.this, "Please enter password", Toast.LENGTH_LONG).show();
        return;
    }

    //if the email, password and username are not empty
    //displaying a progress dialog

    progressDialog.setMessage("Registering Please Wait...");
    progressDialog.show();

    //creating a new user
    mAuth.createUserWithEmailAndPassword(email, password)
            .addOnCompleteListener(this, new OnCompleteListener<AuthResult>() {
                @Override
                public void onComplete(@NonNull Task<AuthResult> task) {
                    //checking if success
                    if (task.isSuccessful()) {

                        createNewUser(task.getResult().getUser());
                        startActivity(new Intent(getApplicationContext(), ProfileActivity.class));

                    } else {
                        //display some message here
                        Toast.makeText(SignupActivity.this, "Registration Error", Toast.LENGTH_LONG).show();
                    }
                    progressDialog.dismiss();
                }
            });
}

private void createNewUser(FirebaseUser userFromRegistration) {

    String username = mUsernameSingUp.getText().toString().trim();
    String emailReg = userFromRegistration.getEmail();
    String userIdReg = userFromRegistration.getUid();

    Map<String, String> newUser = new HashMap<>();
    newUser.put("Username", username);
    newUser.put("Email", emailReg);
    mDatabase.child("users").child(userIdReg).setValue(newUser);

    Map<String, String> usernameIsTaken = new HashMap<>();
    usernameIsTaken.put("Username", username);
    mDatabase.child("takenUsernames").push().setValue(usernameIsTaken);

}

@Override
public void onClick(View view) {

    if (view == mSingUpBtn) {
        checkUser();
    }

    if (view == mRegTv) {
        //open login activity when user taps on the already registered textview
        startActivity(new Intent(this, LoginActivity.class));
    }

}
}

mDatabase.child("usernameTaken").addValueEventListener(new ValueEventListener() {
        @Override
        public void onDataChange(DataSnapshot snapshot) {
              for(DataSnapshot IDGenerado : snapshot){
                   String usernameTaken = IDGenerado.child("Username").getValue(String.class);
                   if(username.equals(usernameTaken))
                      flag = true; // Indica que ya fue tomado el username
              }
              // si el flag nunca se marco como verdadero, entonces no se ha tomado el username
              if(!flag)
                 registerUser();
              else
                 Toast.makeText(SignupActivity.this, "Username already taken", Toast.LENGTH_LONG).show();
        }

        @Override public void onCancelled(DatabaseError error) { }
    });