You can make this using the online/built-in Tutorial, or follow the directions below...
Choose the balloon or ball sprite (needs to be one with a few different costumes/colors).
Change the white background... go to Backdrop >> Edit...
Select the blue sky backdrop OR fill the background with a color...
Choose a color (first or later), choose the Paint Bucket and make sure to Convert to Bitmap... then click in the stage to set the color.
Variables >> Make Variable >> give it a name
When Flag event >> in a forever loop, have balloon go to a random position every 1 sec.
To make it more interesting, have the balloon change color every second (inside the same Flag event
want the score to go back to zero every time you click the Flag...