The first milk is called colostrum and is full of nutrients clear liquid. You'll have this then in between days 3-5 your milk will come in. You won't have to give the baby formula colostrum is exactly what the baby needs at first
When they are born they can suck on your breasts which is what makes your milk come in. They get colostrum from it which has lots of nutrients. The first day their little stomachs are very tiny. And the next few days their tummys are still very tiny anyway.