How to use tmobile in Poland?

I live in illinoise & have an iPhone 4 using tmobile, I'm going on vacation to Poland for 3 weeks.

I was wondering if there was a way I could text others while I'm there.
Do I buy a monthly plan out there?
Do I have to buy a new sim or phone?

I'm clueless, I just want to be able to text out there using my phone, Help

Well you could simply bring your phone to Poland but you would have to pay roaming fees (which are probably high).

I suggest you request an unlock from T-Mobile, I'm not sure if they will unlock your phone permanently or just for those 3 weeks. Anyway, you'll be able to use your iPhone with any Polish carrier.

Prepaid SIM cards are really cheap here. I don't know about T-Mobile, but if you choose Play (it's a Polish carrier), for 30PLN ($10), you'll have around 60 minutes of talk time and 500mb of Internet. There's also this new carrier called NJU Mobile which is cheap from what I've heard.

Ask for a "microSIM T-mobile starter" or a "microSIM Play starter" in a kiosk, or gas station or pretty much any store. They'll give you a prepaid SIM card. You might have to top up the account after you buy the card but it's still the easiest and cheapest option.

You can usually buy a SIM card in European countries that allows you to use your phone's features. T-Mobile also has selected international affiliates that will handle your call, but the rates are really high.
If your phone was from T-Mobile, you need to have it unlocked to use a different SIM card. If you phone T-Mobile customer support and tell them you are going to Europe for three weeks, they will email you an unlock code so you can use a foreign SIM.
Another way to stay in touch, is to open a Skype account with Skype out. Open the account in the US, and when you are in Europe, you can phone US Numbers for $0.012 per minute. SMS is a little more expensive, ten to twenty cents per message. Skype will work on most computers and tablets, and selected phones.
Older T-Mobile phones had an Internet Calling function, that was free if you had a European wi-fi connection. I don't think your Iphone has that capability.