You would need to supply a canteen room (with serving tables) where prisoners can access it on foot, or be let through secure jail doors to it, with it also being a valid room by having tables, chairs & serving tables to recieve the food. All the food is supplied by the kitchen room (requiring utilities to power/water plugged in directly) that requires to be accesed by cook staff but doesn't *nessecarily* have to be connected directly to the canteen building.
Later on you can assign canteens to cater to specific areas, but at the start they will automatically assign to whichever one is closest. I thoughly recommend playing the tutorial as it covers much of this and some additional concepts. Keeping prisoners fed is a important need on their priorities but it reduces their overall ability to rebel if they are hungry, vs being fed and strong enough to fight.