After birth, the production of breast milk is 98% effective in preventing pregnancy if it is done every 4 hours (this includes feeding at night). Weaning the baby off milk is a natural trigger to start your periods again.
If you feed but not as often then a progesterone only (mini) pill is recommend as a form of contraception. Alternatively condoms and other non-hormonal methods can also be used.
The combined pill is known to reduce the flow of breast milk and is not recommended when you are breastfeeding. It can be started after you have stopped breast feeding.
If you decide not to breast feed then you need to start the pill 21 days after the birth date. Additional protection is needed for 7 days if you start later than the 21st day to allow the hormone levels build up.
You should always check with your doctor before starting the contraceptive pill or switching between pills.