Take classes at a local YMCA if they offer such a thing. Check around in your city as to where you can get classes to learn how to handle a rapier. It may take a little while, but the more you know about how to handle a rapier, particularly in a crowded area, the safer you and everyone else around you will be. The art of fencing is exciting but does require agility and endurance to continue in any competitions.
Learn from a fencing master first. If you are already in a ballet company and a ballet comes up in the season requiring fencing knowledge, the director will (or should) arrange for a fencing master to come in and work with all the men involved in those scenes. It is very much like the way actors in movies have to learn all the fighting techniques before they can even go shoot the scene. The reason behind this is the realistic look required, but safety is first and foremost for all concerned.
Pay close attention to everything your fencing master tells you about how to use the rapier. Your rapier will have a safety guard on the end to avoid serious damage, but there is the element of danger when a rapier is swung wider than usual and catches someone in the face. Never fool around with a rapier while in class. Wear all the protective gear required while learning to fence.
In a ballet, each step and movement with the rapier is choreographed, and the movements should never be changed. Each move, however dramatic, is a thought-out step process from one rapier thrust to the next. Dancing in between fencing is challenging, but most professional male dancers are very adept at picking up the movements. After all, fencing is just like a dance in its own ballet.