Breastfeed first, everytime, so that your milk supply doesnt drop.. and then you can give baby small amounts, 30-60ml.. if s/he wants more you can tell cause they either cry or search for more. If they are full they will refuse the bottle. Just don't force the bottle to the baby and let them guide you. But please insist on breastfeeding, it is the very best for the baby. You can try ways to increase your milk supply, like feeding frequently, pumping, drinking milk, eating oats.. etc.
Wish you luck.