If you want to take a taxi, you must either call and book a taxi or walk to the nearest taxi park; catching a taxi on the street is not recommended.
To avoid unpleasant situations, use the official taxi - a white car with «Servizo Pablico» lettering, lit up sign and a phone number. If you wish to save money, ask the driver to start the meter. 
Step-by-step instruction to call a taxi: you call an agency and they tell you the taxi number which is displayed on the doors of the taxi.
It is recommended that you carry your hotel business card with you in case the driver cannot understand where you have to go.
Taxi prices in Italy
Call – 2 €
Getting into a taxi + 3 km ride - 7-8 €
Over 1km – 1 €
Luggage fee – 1 € per space
At night (22:00-06:00) - 30% more expensive 
When you call for a taxi, you are responsible for paying taxi’s trip to your location, so keep that mind and try to call the nearest taxi park to avoid paying too much.
And the last tip: change large bills before getting into the taxi because taxi drivers insist that they don’t have change.