Two ways of doing this...

1. Logic circuits.
2. Programable controller (e.g. PICaxe)

Logic circuit is quite straight forward...
Just a 7 segment counter really.
PICaxe and similar requires some prior knowledge really although is dead easy.
Youll need a debounce circuit for the switch... look up switch bounce. Think about different ways of sensing the ball (touch pad is only one).
How to reset the score? What to do with the ball? Maybe the system should place the ball back at the start somehow.