In what way can I make a GUI Inventory?

I would like to be able to have interactable objects that I can pick up and go straight to the inventory(pause menu). ie: ammo, health items, weapons, keys, etc. I've been trying to figure out a way to have it so that it is a popup window, but I have no idea how to make it so that i have item textures that are all a specific size that snap to a grid and have limited space within the inventory. The partial script I have so far:

 var customStyle : GUIStyle;
 private var windowRect : Rect = Rect(Screen.width/2, Screen.height/2, Screen.width, Screen.height);
 // Would it be an Update function or OnGUI or something else to start? I have no clue what I am doing.
 function Update()
 {
     if(Input.GetKeyDown(KeyCode.I))
     {
         if(Time.TimeScale == 1)
             Time.TimeScale = 0;
         GUI.Window(0,windowRect,WindowFunction, "Pause Menu");
     }
 }
 function WindowFunction()
 {
     
 }

I would also like to have it carry over from scene to scene. Does anyone have any ideas that can help?