Utique sits about 35 km northwest of Tunis, off the GP8 road towards Bizerte. There is no direct public transport to the site itself.
The simplest option is to hire a taxi from Tunis for the round trip — agree on the fare in advance, including a wait of an hour or two at the site. A louage (shared minibus) from Tunis to Henchir Bou Chateur or to Utique-village will get you close, but you will still have a couple of kilometres to walk to the archaeological park.
The site combines naturally with a day trip to Bizerte or with Carthage on the way back into Tunis. If you have a rental car, parking is informal — pull off the road near the entrance.