A number of airlines fly to Florence, including British Airways, United, Lufthansa, Air France, American Airlines and Alitalia.
If you are an EU citizen, you do not need a visa. If you are a non-EU citizen you will need a valid passport and depending on your country of origin, you might need a visa; you could obtain a visa through the Italian Consulate nearest to your residence.
Italy has an excellent public transport system of both buses and trains, both within the cities and between cities. Italy has many regional airports and the main domestic carrier is Alitalia. Car hire is readily available and there is an extensive network of motorways.