It isn't easy. Supply can be very difficult to build up and maintain. You'll need to use a breastpump until your baby is mature enough for direct breastfeeding, and sometimes for longer. If your baby is stable and relatively well, breastfeeding can be gradually introduced from about 32-33 weeks. You'll need to begin expressing by the day after your baby's birth, and even earlier if possible.
Some mothers prefer to begin with hand expressing.
A nurse or lactation counsellor should show you the proper way to express either by hand or with a breastpump. Once your milk has come in, you'll need to begin using a good breastpump. The pump may feel uncomfortable at first, so try to use a low setting. In time, you will be able to increase the speed and the suction setting of the pump. For the first day or two, before your milk comes in, you'll should pump for about 5 minutes every three hours during the day, and once during the night.