READ THIS BEFORE FLYING

(ArduCopter's source)

When flying your multicopter, you need to keep in mind that it flies much differently than a winged airplane. In an airplane, lift is generated by the wing and you still have control surfaces for maneuvering if the motor were to stop. ArduCopter, as well as other quadcopters, generates lift solely from the thrust of the motors. If your battery were to go dead, you would have no lift and no control over your ArduCopter. It will flip over and crash without a gradual warning. During first flights, fly only short durations until you know your battery consumption.

It is HIGHLY recommended you purchase a Battery Voltage Monitor such as the one found here for 3S LiPo batteries. The two wires can be soldered to the P-PCB directly, or you can connect to the balancing plug of your battery.
The one linked above has a buzzer and LEDs that function as follows:

Arming the Motors

Before arming the motors, make sure all people and objects are clear of the propellers. Arming is done by applying Full-Right Yaw and No Throttle. On a Mode 2 transmitter, this means the left stick would be Down and to the Right. Disarming is done by applying Full-Left Yaw and No Throttle. On a Mode 2 transmitter, this means the left stick would be Down and to the Left. Both the arming and disarming positions need to be held for a few seconds. This is a safety mechanism to prevent accidental disarming during flight and accidental arming during transportation.