All the main rental companies have their branches in Tunisia and you can easily rent a car at any airport. If you arrive from Italy by a ferry it‘s also possible to easily hire a car. The cheapest car you can hire starts from about 50 dinars a day. Be aware you will get an empty tank on hire, just enough to get to the nearest gas station. If you plan on going to southern Tunisia and roam in the desert you should rent a 4x4 offroader. Mind you, offroaders are double the price.
Surprisingly, the roads in Tunisia are quite decent, most of them are paved. Drivers are mainly ignoring the rules and driving way to fast. Mind you, don’t try this for yourself, because the fines for speeding are very heavy. The methods of speed detection are quite antique, so you won’t be able to bargain your way out of it. The traffic signal and most of the signs are ignored, so be careful – if the green light is on don’t just drive carelessly trough.