The circuit for the push button is fairly straightforward—connect one of the button's legs to the 5 volt power supply, and the other to one of the digital pins on your WiFi shield. In our diagram, we've chosen pin 8. Then, connect that same leg that is wired to pin 8 to ground through a pull down resistor.

When the button in this circuit is pressed, pin 8 will be connected to power, and will read HIGH. When the button is left open, pin 8 is connected to ground, and will read LOW.