Count the Score

You need a score variable (probably static), that you increase every time you kill an enemy and then display it on the GUI.

Add this to your enemies:

 function OnDestroy() {
     score.score += 10;
 }

And then this would be score.js:

 static var score : int = 0;
 
 function OnGUI() {
     GUI.Label(Rect(X, Y, width, height), "Score:" + score);
 }