Dead easy. There are usually two direct trains per hour, from Pisa Centrale to Firenze Rifredi, with journey times ranging from 58 minutes to 1 hour 15 minutes.
The best journey planner, for all European rail travel, is the website of the German railways, Deutsche Bahn:
http://reiseauskunft.bahn.de/bin/query.exe/en
The Trenitalia website seems to have ironed out some of it's many problems recently. If you want to try it, it's here:
http://www.trenitalia.com/en/index.html
Italian rail services are generally very cheap (by UK standards). If you want to pre-book, try Trenitalia's UK partner, here:
http://www.italiarail.co.uk/tickets/IR/IRUK.as p?AFF=ITR
Chris