Mature breast milk typically comes in 2-5 days after birth (and may take even longer if you had a C-section, traumatic delivery, or if this is your first baby). Give yourself grace! This is one of the most common concerns of the mothers we see. Even before the mature milk comes, you are making good colostrum (thick yellow drops) that has wonderful antibodies and fat for your baby.
Your pediatrician will monitor your baby with weight checks and physical exams several times in the early newborn period to make sure they are staying safe and healthy while your milk supply comes in. If it is found that supplementation with formula is necessary in the short term to keep your baby healthy, you should pump or hand-express milk during the formula feeding times (every 2-3 hours). The frequent stimulation of the breast will tell your brain to keep trying to make milk.