I am flying into the Rome airport from the United

you need a passport of course.  also, make sure you have proof of vaccination available.  the only train at FCO is called the Leonardo, it’s across from T3.  This train takes about 30 minutes and only goes to Rome Termini train station.  From Termini, you can grab a high speed train to Firenze.  Besides TrenItalia Frecciarosa, you can also book on Italo.  I like using Italo.  GL

you need a passport of course. and you can take a cab or taxi to the train