It is the up and down motion that compresses the areola and begins the release of milk in the baby's mouth. The baby's tongue should be on the underside of the nipple as this helps to push the milk out also. Mothers with especially large breasts may want the place a finger between the base of the breast and the baby's mouth to allow the baby to get air to its nose. When it's time to take the baby off the breast, break the suction by sliding a finger into its mouth. If you try to pull, the baby will automatically clamp down and tighten the grip on your nipple.
Most mothers offer each breast at every feeding, alternating which breast they begin with.
Pinning a safety pin to your nursing bra, on the side that was last given, helps you to keep track of which breast was offered first at the last feeding. You will experience very sore nipples for the first few days, until the kinks are worked out and your nipples become adjusted. Offering 5 minutes on each breast the first day, doubling the time the next day and then finally offering 15 minutes or whatever your baby needs after that, will establish a routine on each side. Once you've been nursing for a while and your letdown reflex is established, your baby will get most of the milk it needs in the first five minutes of nursing. Since breastfed babies swallow less air they will probably only need to be burped twice in a feeding, once after the first breast and then after the feeding.