Moving object in a bounday

Dear corona people,

I am new bee in this field. I am building a game in which I need couple of balls should arise in the screen one by one and they will be moving randomly in to the screen. They will never leave the screen/go out of the screen.

If we touch any balloon that balloon will disaper.

Please help me how to do this. I have tried many things but not a single things works, and I am in a great pain now.

Thanx in advance. If any body can give me any sample code than it will be a great help.