When serving, you must have one foot on the floor, within the service box (inside, not touching the line) when you strike the ball.
If this rule is not adhered to the referee is supposed to say "Fault". The player will then ask what was wrong, and the referee will say "Foot Fault", so we suggest you just say "Foot Fault" in the first place to avoid unnecessary delays and confusion.
Foot faults only apply to recreational players - professional players are allowed to have their foot outside the box or in the air.
Well, the latest rules say that you should call "fault", but everyone we know thinks you should call "out" just like any other time the ball goes out.