I have to very much agree with Stacy. Breeding large dogs (or any dog for that matter) should not be taken lightly. St. Bernards are prone to a whole host of devastating genetic conditions, including bad hips and elbows, knee issues, eye problems, along with several very, very sad types of cancers, like osteosarcoma.
At this point, I would recommend getting in touch with a well educated breeder (which may be the case with the people you bought this dog from...though often breeders don't want their dogs bred without proving themselves in a show ring) and ask to shadow that person. Chances are, most breeders will tell you these same things, but perhaps with more frustration. Backyard breeding really does a lot of damage to the breeding world, and most of the time is completely unnecessary. Remember, breeders that do things the RIGHT way often spend more money ensuring the health of the dogs they are breeding and the future of those pups than they make on a litter. If you are intending to breed these dogs to make money, you are not breeding for the right reasons.