You should know how to create Movie Clips and name them.

When using the code below, apart from debugging (bugs embedded), you must comment each line to show understanding.

stop();

startGame.addEventListener(MouseEvent.CLICK, startTheGame);

function startTheGame(e:MouseEvent) {
         gotoAndPlay(2);
   
     } //eof startTheGame()

On Frome 2 of Actins layer, write this code:

stop(); //what does it do on Frmae 2?

//make the paddle move with the mouse
   paddle.addEventListener(Event.ENTER_FRAME, moveThePaddle);
   function moveThePaddle(e:Event){
        paddle.x = mouseX;
        if((paddle.x - paddle.width/2)<=0){
              paddle.x = paddle.width/2;
        }
        if((paddle.x + paddle.width/2)>=600){
              paddle.x =  600-paddle.width/2;
        }
   }    

//move the ball
         var ballXspeed :Number = 10;
         var ballYspeed :Number = 5;
         var score :Number = 0;
         var lives :Number = 3;
         addEventListener(Event.ENTER_FRAME, moveTheBall);

         function moveTheBall(E:Event){
              ball.x += ballXspeed;
              ball.y += ballYspeed;
              if(ball.x <=0 || ball.x >=600){
                   ballXspeed = - ballXspeed;
              }
              if(ball.y <=0){
                   ballYspeed = - ballYspeed;
              }
              if(ball.y >=400){
                   lives --;
              }
              if(ball.hitTestObject(paddle)){
                   ballYspeed = - ballYspeed;
              }
              if(ball.hitTestObject(brick)){
                   score += 20;
                   brick.parent.removeChild(brick);
                   scoreDisplay.text  =String(score);
                   removeEventListener(Event.ENTER_FRAME,moveTheBall);
              }
         }

Here is the last few lines of code above: 

     if(ball.hitTestObject(brick)){
             score += 20;
             brick.parent.removeChild(brick);
             scoreDisplay.text  =String(score);
             removeEventListener(Event.ENTER_FRAME,moveTheBall);
     }

These need to be changed when more bricks are added.
Suppose there are 8 bricks all together, then there would be 8 blocks of code like that.
If 10 brciks, then 10 blocks of code will be required.
To simplify, we use Array, a new data type, to hold all the bricks.

Apart from the brick in the frame already, add 7 more.
the use the following code to replace the above block of code 

     var bricks :Array = new Array();
     bricks.push(brick1); // the brick1- brick7 are the names 
     bricks.push(brick2); // of bricks on the screen
     bricks.push(brick3);
     bricks.push(brick4);
     bricks.push(brick5);
     bricks.push(brick6);
     bricks.push(brick7);
  
     var i :int = 0; 

     for (i = bricks.length-1; i>0; i--){
         if(ball.hitTestObject(bricks[i])){
             score += 20;             
             scoreDisplay.text  =String(score);
             bricks[i].parent.removeChild(bricks[i]);
             bricks.splice(i);
         }
         if(bricks.length == 0){
             //level over
             trace("Level Over");
             messageDisplay.text = "Level Complete!";
         }
     }

//move the ball
     var ballXspeed :Number = 10;
     var ballYspeed :Number = 5;
     var score :Number = 0;
     var lives :Number = 3;
     var bricks :Array = new Array();
     bricks.push(brick1); // the brick1- brick7 are the names of
     bricks.push(brick2); // bricks on the screen
     bricks.push(brick3);
     bricks.push(brick4);
     bricks.push(brick5);
     bricks.push(brick6);
     bricks.push(brick7);
     var i :int = 0; 

     addEventListener(Event.ENTER_FRAME, moveTheBall);
     function moveTheBall(E:Event){
               ball.x += ballXspeed;
               ball.y += ballYspeed;
               if(ball.x <=0 || ball.x >=600){
                    ballXspeed = - ballXspeed;
               }
               if(ball.y <=0){
                    ballYspeed = - ballYspeed;
               }
               if(ball.y >=400){
                    lives --;
               }
               if(ball.hitTestObject(paddle)){
                    ballYspeed = - ballYspeed;
               }
               for (i = bricks.length-1; i>0; i--){
                    if(ball.hitTestObject(bricks[i])){
                         score += 20;
                         scoreDisplay.text  =String(score);
                         bricks[i].parent.removeChild(bricks[i]);
                         bricks.splice(i);
    }
               if(bricks.length == 0){
                //level over
                      messageDisplay.text = "Level Complete!";  
                      //you need to draw a TextField on the screen and 
                      name it "messageDisplay" 
               }
     } 

The above method of checking each element in the array to test if it hits the ball usually works but may have problems. Here is an alternative:

//set up the listeners, let the computer do the hardwork
        for (i = bricks.length-1; i>=0; i--){
             bricks[i].addEventListener(Event.ENTER_FRAME, checkHits);
        }

//this is the event handler
        function checkHits(E:Event){
            if(ball.hitTestObject(MovieClip(E.target))){
                score +=20;
                MovieClip(E.target).parent.removeChild(MovieClip(E.target));
                E.target.removeEventListener(Event.ENTER_FRAME, checkHits);
                scoreDisplay.text = String(score);
                numRemoved ++;
            } else{
            
            }
        } //eof checkHits()

       if(numRemoved == bricks.length){
             messageDisplay.text = "Level 1 completed!";
             ......... (see below)
       }

  if(score >=30){
		messageDisplay.text = "Level 1 Complete!";  
		//remove all remaining bricks
		for(i = bricks.length-1; i >=0; i--){
			bricks[i].parent.removeChild(bricks[i]);
		}
		//bricks= null;
		ball.parent.removeChild(ball);
		paddle.parent.removeChild(paddle);
		//paddle.addEventListener(Event.ENTER_FRAME, moveThePaddle);
		paddle.removeEventListener(Event.ENTER_FRAME, moveThePaddle);
		//addEventListener(Event.ENTER_FRAME, moveTheBall);
		removeEventListener(Event.ENTER_FRAME, moveTheBall);
		//
		gotoAndStop(3);
					
	}