The canter is a three time gait with the horse placing one leg, then two together (a diagonal pair) and then the remaining leg on the ground. It is more comfortable than sitting trot with a rocking motion although the faster speed can be intimidating to the novice rider at first.
The horse will "lead" with one front leg, that is one front leg is thrown further forward by the horse than the other. The correct lead is when the horse throws the leg nearest the centre of the school or arena further forward than the leg nearest the outside of the school or arena. The horse finds it easier to balance on the correct lead when cantering around in circles.
The aids for the canter are different for the walk and trot in order to "tell" the horse which leg to lead with. Whilst in sitting trot, to ask for canter the leg nearest the outside of the school should be moved gently back slightly whilst the leg nearest the inside of the school remains in the normal position. With the legs in this position, sit up straight and gently squeeze or give a small kick to ask the horse to canter.
Avoid any tendency to hold onto the reins too tight or to tip forward as this will most likely cause the horse to come back to trot.
As in the walk, the horse nods it's head in canter and so you should keep your elbows relaxed allowing your hands to move forward and back with the movement of the horse's head.
The back should remain soft and supple to allow your hips to rock with the horse's movement when cantering. A stiff back will make the canter uncomfortable for both horse and rider.