Well, you need to begin by making sure you are comfortable and collected at the walk and trot before you attempt to canter, but when you do, if you want to ask for the left lead (cantering to the left) you will squeeze your left leg and heel (inside) at the girth, and give pressure with your right leg and heel (outside) just behind the girth. This cue with the right leg further back (kind of a scraping backwards motion) signals to the horse to start the canter with his right leg first, because the leading leg (left in this case) will be the last to hit the ground in the 3-beat canter (or lope)
Hope this helps!