Resolutions

Hi guys, a have i little doubts about the resolutions. I've created my application, and then i run it on 800x600, the GUI elements was "messed", how can i do to make one code to care about all resolutions?

Even i try to do this, i failed D=

 void Start() {
 screenX = Screen.width/2;
         screenY = Screen.height/2;
 }
 
 void onGUI() {
       public void createBox(float left, float top, float width, float height, string content) {        
         GUI.Box(new Rect((screenX-screenX)+left, (screenY-screenY)+top, width, height), content);    
     }
 }

Thanks guys!