Making dynamic login screen

Hi,

I'm a complete newcomer to unity, but I'm not a bad programmer and I've done a fair bit of work with 3D modelling so you don't need to be too gentle :)

As an 'intro' project, I am making a little client for a javascript based webgame. I want to make a login screen, where it shows a background image and has 2 buttons on it, Login and Sign Up. When one of these is clicked, I want the buttons to disappear and a form to appear, either to sign up or to login.

I have the buttons and background set up, but could someone point me towards a way to switch the components nicely? At the moment I have the following code which makes the buttons vanish, but no way to create the new form. I get the feeling I should be making prepared sections that I can swap in and out, but not really any idea where to start.

 public class GUITest : MonoBehaviour {
 public Texture btnTexture;
 private bool showButtons = true;
 private bool showLogin = false;

 // Use this for initialization
 void Start () {
 
 }

 void OnGUI () {
     if (!btnTexture) {
         Debug.LogError("Please assign a texture on the inspector");
         return;
     }

     if (showButtons) {

         // Make the first button. 
         if (GUI.Button (new Rect ((Screen.width / 2) - (btnTexture.width / 2), (Screen.height / 2) - 100, btnTexture.width, btnTexture.height), btnTexture, GUIStyle.none)) {
             Debug.Log ("Clicked the login button");
             showButtons = false;
             showLogin = true;
         }
     
         // Make the second button.
         if (GUI.Button (new Rect ((Screen.width / 2) - (btnTexture.width / 2), (Screen.height / 2) - 40, btnTexture.width, btnTexture.height), btnTexture, GUIStyle.none)) {
             Debug.Log ("Clicked the sign up button");
             showButtons = false;
         }
     }
 }
 
 // Update is called once per frame
 void Update () {
     if (showLogin) {
         ShowLoginForm ();
     }
 
 }

This is to show the login form. Doesn't work at the moment as apparently I can only call GUI elements from onGUI, but here it is to give an idea of what I'm aiming at.

 void ShowLoginForm() {
     Debug.Log ("Creating form");
     int centreX = Screen.width/2;
     int centreY = Screen.height/2;
     GUI.Box(new Rect(centreX-150, centreY-100, 300, 400), "Login");
     Debug.Log ("Created box");
     GUI.Label (new Rect (centreX - 80, (centreY - 100 + 25), 160, 50), "Username");
 }

Ideally I would be looking to add transition effects to the change from buttons to form but that can come later.

Thanks!