How to get train from Milan airport to Florence.
No direct experience, but perhaps this will help:
https://www.milanomalpensa-airport.com/en/from-to/by-train
Per the above link, the train runs from MXP T1/T2 to Milan Central.
There are multiple trains per day from Milan Central to Florence (Firenze)
There is a train station at the airport (I'm assuming you mean Malpensa?) Since there is no guarantee of on time arrival or speed of clearing the airport, buy your ticket once you've arrived. You can do it via the Trenitalia website or app, or you can buy it from a ticket machine or manned ticket desk. There are no direct trains. You will have to change trains at least once, in Milan.
This is the website: https://www.trenitalia.com/en.html
You need to use the Italian place names: Malpensa Aeroporto, and Firenze S. M. Novella