From the street. There's an entrance to the left and a counter to the right
From the Pompeii website:
Although the excavated part in Pompeii (which amounts to two-thirds of the old urban area) has 89 thermopolia, small cook-shops where hot food was sold, it does not mean that the wealthy owners of large atrium houses used to dine out. The lower classes frequented such places. When passing through Pompeii, one can see many dwellings and shops within the large houses, which overlook the street and often consist of a single room. Craftsmen and merchants lived and worked here, often living with their families on the upper floors. These homes rarely have a kitchen. Therefore one would dine out, in one of the many thermopolia that offered hot food.
This is one of them: