A 240 volt device does not use a separate neutral. The white wire of the 12/2 is wrapped with some black electrical tape and connected to the breaker along with the black wire. The ground goes to the grounding bar.
It's recommended to shut of the main disconnect before you connect the circuit breaker. Be aware that even with the main disconnect 'off' the main service wires feeding the panel are still LIVE.
"If you think education is expensive - try ignorance." (Bok's law)