ATmega328p simple shortcircuit button
The problem here is you set pull-up on D0. So even if button is not pressed it will be at high state because of pull-up. You would need pull-down for this to work (Arduino does not have it).

The pullups are useful if you want to connect pin to GND when button is pressed. When button is not pressed it will have defined state (HIGH because of pull-up) and when pressed it will be pulled LOW by button. You can imagine pull-up as a resistor to Vcc.

So for this to work I would put the other end of button to GND and it will work (One end to D0, second on GND). Condition in code should be altered (a == 0) but otherwise all can be the same.

Other option is to connect external resistor in range 1K-100K Ohm between pin D7 and GND and turn off pull-up in code. That resistor would serve as pull down.

Just one more note. If you are not using other pins it is the best to leave them as inputs. Setting an "unused" pin as output and setting it to HIGH can damage the pin if you accidentally connected it to GND (because in previous sketch you experimented something).