South Sudanese food is simple, filling, and built around sorghum, millet, and whatever protein is available. Don't expect much variety, especially outside Juba.
Asida is the staple — a thick porridge made from sorghum or millet, similar to ugali or fufu elsewhere in Africa. It is eaten by hand, usually with a stew of meat, dried fish, or vegetables. Goat is the most common meat; beef is eaten but cattle are too culturally important to slaughter casually. Dried fish from the Nile is common and adds a pungent, savory note.
Kissra is a thin, slightly sour flatbread made from sorghum, common in Juba and among communities with ties to Sudan. It goes with almost everything. Ful medames — stewed fava beans with oil and lemon — is a Sudanese influence that has crossed the border and become common in Juba, usually eaten for breakfast.
In Juba, you can eat reasonably well. Restaurants aimed at NGO workers and expats serve grilled meats, Ethiopian food, and a mix of East African dishes. Prices are high by regional standards. The Konyo Konyo market area has street food — look for grilled goat and flatbread in the evenings.
Alcohol is available in Juba, mainly beer (local Nile Special and East African imports) and spirits in restaurants and bars. Outside Juba and in conservative Muslim areas, it is less available.
Water is not safe to drink from the tap anywhere. Buy bottled water and treat any water you collect yourself.