Dogs reproduce sexually. A male dog deposits sperm inside the female where fertilization takes place. The mechanics of it are similar to most mammals. Female dogs give live birth to a litter of puppies. You can find more information here: animalpetdoctor.homestead.com/Reproduction.html
1 Additional Answer
Dogs reproduce when a female dog is in heat. Depending on the dog it can go into heat from 2 to 5 times a year. You will introduce a male dog at the time she is in heat.