It should sit on top of your head in a level position. It should not rock forward and back or from side to side. The helmet straps must always be buckled. It can't protect you if it flies off during a crash. If you have been in a crash and your helmet has been hit, don't use it again.
Small cracks can weaken the shell and the foam can be crushed.
And be sure to purchase a helmet that is certified to meet or exceed the safety standards developed by the U.S. Consumer Products Safety Commission. Learn the rules of the road, and obey all traffic laws. Ride on the right side of the road with the flow of traffic, not against it. Use appropriate hand signals to indicate your intentions. Observe all traffic signals, including stopping at all stop signs and stoplights.