Getting legally married in Italy can be a bit of an Odyssey. What may seem obvious in your country isn't so in Italy. The most obvious answer is to choose a lovely city hall, follow directions, and set a date. You will have to pay a fee to the municipality that is normally around €400. Some more famous city halls, such as Florence's Palazzo Vecchio can cost thousands of euro for a ceremony lasting about 15 minutes---but come on, it's Palazzo Vecchio.
Another option is to legally marry directly at the venue of your choice---there is a catch. Not all venues are authorised for legally binding ceremonies. The ceremony is still performed by an official from the municipality in which the venue is geographically located and the venue has to be registered to do so. Make sure you check with your wedding planner or with the venue directly to guarantee this type of ceremony is possible which will help avoid disappointment later on. You will have to pay an extra fee to the venue, similar to that of the municipality in order to legally marry at your chosen venue.
Same-sex unions have been legal since 2016 in Italy and the procedure is the same as above.