a question of "how to start" is still pretty vague.
Do you need help buying a computer to use?
Do you need help installing java?
Do you need help in writing, compiling, and testing a "Hello, World" program?
Do you need help with the math?
Assuming you only need help with this specific program, you would generally start with pencil and paper. Work through what your program would look like and print out on the screen. Figure out various methods that may be helpful...like generating a random number, or how to run something N times. Figure out what kinds of things you need to keep track of (total score, number of balls shot, etc). do all this ON PAPER.
Only when you have a good feel for the various components should you start writing a line of code.
There are only two hard things in computer science: cache invalidation, naming things, and off-by-one errors