Getting there the smart way
If you have just a few days to spend in Tuscany, and you are based in Florence, your best option to visit Pisa is a one-day trip..
Pisa is well connected with Florence by public transport, the recommended way to get there is by train. More or less every 30 minutes, a regional train departs from the station of Santa Maria Novella (central station of Florence) heading to Pisa. Some trains are direct, and will take you to Pisa Centrale (the central station of Pisa) in 50 minutes, others stops in several stations along the way and will take a little bit longer (the official website for time table and prices is www.trenitalia.com).
Once you get to Pisa you can walk (north) across the beautiful city center for about 35 minutes reaching the Torre Pendente (Leaning Tower) and the beautiful Piazza dei Miracoli (Square of Miracles). A bus line called LAM Rossa (red line) is also available, and connects the central station to the Leaning Tower (ask the driver, or wait for the local people to prompt you where to drop off the bus – They will!).
If you are going for a very quick visit, and just want to get juiced in it, you should take a train stopping in Pisa San Rossore (a secondary station located north of Pisa Centrale) that is just 5 minutes walk from the tower.
Other options to get to Pisa and back to Florence are the long distance bus that connects the two airports (they change often, so check online to get updated information), or the widely used ride sharing service called BlaBlaCar (download the app and hitch rides to everywhere traveling on common people's car).