Puberty begins at an age which varies from race to race and according to the size of the dog. It generally happens at between 6 and 12 months.
In males, it features changes in behaviour: The dog cocks its leg to urinate, and is attracted by the scent of females in heat.
In bitches, reproduction is cyclical. Every 6 months on average, the bitch will go into "heat". This phase, corresponding to ovule maturation and ovulation, features a swelling of the vulva, uterine bleeding (visible at the vulva labial commissure) and attraction to males. N.B.: this is not your dog´s "periods", but normal bleeding for this species prior to ovulation.
During the average of three weeks in heat, the bitch is fertile for only some 48 hours.
The optimum coupling time for having puppies tends to lie between the 8th and the 25th day, although two thirds of bitches are "ready" from the 10th to the 15th day of heat.
Your vet can help you identify the right time.