Orvieto is on the main Rome-Florence rail line (the direttissima), making it easy to reach from both cities. Trains from Rome take around 75 minutes; from Florence about 90 minutes. The station is at the base of the cliff; take the funicular up to the town. By car, Orvieto is just off the A1 motorway (exit Orvieto), about 90 minutes from Rome and 2 hours from Florence.
Parking in the old town is limited and expensive. Use the Parcheggio Campo della Fiera car park at the base of the cliff, which connects to the funicular, or park in the free lots at Orvieto Scalo (the lower town) and take the funicular up.