1) The first requirement of using a cell phone overseas is to have a phone that uses GSM (Global System Mobile) technology. T-Mobile phones, as well as some Cingular and AT&T Wireless phones are equipped with this technology. 
 
2) You need the "unlock codes" to unlock your phone and use it overseas. For $5, you can get the codes from thetravelinsider.info. There are several other sites that also list codes.  
 
3) Then, you will need to buy minutes from the company for where you are traveling. You can buy pre-paid minutes from telestial.com. The company will send you a chip that you use to replace the current chip inside your phone. 
 
4) You're ready to go!