+1 to all above comments..
You cannot take a CDMA phone to a GSM carrier, UNLESS the phone is a World Phone (meaning, it has a dual radio) and SIM card slot. I believe you would have to flash the phone's software to make it compatible, but it's been so long I can't remember.
If you were given a Droid, lucky you, but it's useless to you if you are unable to swap carriers (under contract with TMO, phone's ESN is not clean, etc.). My advice? Ensure the ESN is clean, and sell it on Craigslist along with whatever accessories you have for it for a couple hundred, take that, and buy an Android phone on TMO.