Position the ball
At the start of the game we want the ball to start at the top center of the screen. We can do this by setting the X and Y coordinates of the ball.
The X and Y coordinates are like an address, they give the location of a point on the screen. The X number is the point on the X axis which runs vertically across the screen (left and right). The Y number is the point on the Y axis which runs horizontally across the screen (up and down). The very center of the screen is X:0 Y:0. As you move to the right the X number increases, as you move to the left the X number decreases. Similarly as you move up the Y number increases, as you move down the Y number decreases.
We want the ball to start at the top center of the screen which as at the location X:0 Y:170. To do this we add the following code to the ball sprite:
when green flag clicked
go to x  y 
Once you've added this code and entered the correct X and Y coordinates, click on the green flag at the top right of your screen to run the code. Your ball should go to the top center of your screen.
You should test your code after each step to make sure it works. If there's a problem with it then try and find the piece of code that's causing the problem and fix it.'