When buying a lobster ideally choose a live one to ensure it is as fresh as can be. To humanely kill a lobster you will need to drive a knife quickly and smoothly between the eyes, splitting the head in half. This will instantly kill the it. For more information about killing shellfish humanely, visit RSPCA.org.uk.
Removing the meat form the claw takes a little effort but it's worth it as the meat is sweeter and more tender than the tail meat. Cracking the claws can also be done with a lobster cracker but try not to crush the claw completely as this will damage the chances of the claw meat coming out whole.