Chantielle,
The fact that you're with T-Mobile is a good start, however there's one VERY important point you need to check. Check the Manual for your phone handset, to ensure that it's a quad-band model. If you don't have the two Euro frequencies, your phone WILL NOT work in Europe!
I encountered some young travellers from the U.S. last year in Italy, who were puzzled on why their "GSM Phone" wouldn't work. When I checked the phone, I discovered that it was only equipped with the two North American frequencies, so had to break the news to them that they were carrying an expensive paper weight.
I'd also suggest checking the phone charger to ensure that it's designed for operation on 220 VAC electrical systems. There will be a label somewhere on the Charger that states the "Input Voltage" specifications. A Euro Plug Adapter will be required as well.
If you're using a "Smart Phone" that accesses data (E-mail, web surfing), it would be a good idea to disable that while you're travelling.
I'd suggest speaking with someone at T-Mobile that is familiar with their international calling plans in order to get the best travel plan (not all rep's are knowledgeable on that).
Cheers!