Any dog can come into heat as early as 6 month of age, however, you should wait until she is at least 18 months old before breeding her. The problem is that dogs and cats will go into heat before they are fully grown and they are still considered kittens and puppies until they a 12 months old.
When you decide to breed her, consult her vet about breeding and have the vet supervise the pregnancy.
An importance consideration is the size of the sire and the size of your dog. A very large sire will result in large puppies and a difficult pregnancy, this is another area in which the Vet can advise you.
Good Luck, Wayne
ps - There's nothing more fun than a house full of puppies, but be sure to start their dog training as soon as they leave the den and begin to explore.