Here are some elementary instructions to get you novices started:
To go up and down, press the PAGE UP and PAGE DOWN keys respectively or use the Q key to go up and the Z key for going down.
To move forward, push the mouse forward. The helicopter's nose should tilt down and you should then begin to accelerate. To slow down or move in reverse, pull the mouse back towards you. The helicopter's nose should tilt up, causing you to slow down and eventually fly in reverse.
To move right or left, either use the mouse or your keyboard's arrow keys.
Once you've earned your wings, you might be interested in reading or downloading a helicopter tactics guide, courtesy of FlashpointCenter.