Script for the Coin

   function OnTriggerEnter(hit : Collider){
    if(hit.gameObject.tag == "Player"){
    //add a tag to the player called Player, or just
    //replace it with your tag if you already have one
    ScoreSystem.myScore += 1;
    Destroy(gameObject,0.0);
  }
}

Attatch this Script anywhere but name the Script "ScoreSystem" (this is very important)

static var myScore = 0;
var DelayTime = 10;
function Start(){    
myScore = 0;
Invoke("myFunction", DelayTime);
}

function OnGUI(){   
GUI.Label(Rect((Screen.width / 2) + 20,10, 200, 30), "Score: " + myScore);
}

Attatch This Script Anywhere as well

function Update(){
if(ScoreSystem.myScore == 10){ //change the 10 to however many coins there will be
    Application.LoadLevel("YouWon");//Make a scene named "YouWon" and add it to the build
}

I hope this helps. Please review this code and learn from it.