Best and easiest way to get to the Medina in Tunis
First, unless you are traveling in a group don't rent a car in Tunis, the rates are just very high and driving around Tunis is just difficult since the roads are narrow. At the medina, there's also a problem of overcrowding and securing a parking spot is a problem so driving there is not a good option.
Take a taxi to the medina, prices are relatively good and they know how to avoid traffic. From Carthage, for instance, it takes you just about 10 minutes to get to the medina by taxi. Usually costs between $2-5.
If you're on an extremely tight budget opt for the bus or underground train. For the underground train, you'll arrive at Tunis marine train from where you can walk to the medina. Train rides cost $0.35.
You can take the train from Carthage, Marsa, Sidi Bou Said, and La Goulette.