Learning to ride the canter can be a little intimidating for many beginner riders, but once the canter is mastered it often becomes a favorite, comfortable gait.
It's easiest to learn to ride the canter with the help of a good, patient riding instructor and equally patient, relatively smooth horse.
Before you are ready to ride the canter, your seat should be competent riding the horse at other gaits. You should be comfortable at walk and posting trot and be able to sit the trot a few strides at a time. Don't rush yourself! Some riders canter within a few rides, while in Europe it's not unusual for a riding instructor to keep a rider at walk and trot for a year or more!
You may want to ask to canter for the first time on a lunge line. This allows the horse to stay under the control of your instructor so you can focus on your seat. From trot, take a deep breath, make sure your heels are down, give the cue your instuctor tells you to, and allow your hips to swing with the first surging stride of the canter. Try to avoid leaning forward, staying straight up in the saddle will help your seatbones stay deep and secure in the saddle. If you are in a western saddle you may want to hold the horn the first few times, or in an english, the mane, but you should not allow them to become habit.
Learning to ride the canter is not difficult, but is intimidating to many riders. As you become comfortable in the canter you will learn to tip your pelvis upwards and crunch with your abdominal muscles with each stride to push your horse into the bridle and gain more and more control.