Before choose you cellphone network,you should know about GSM and CDMA.
CDMA(Code Division Multiple Access) is a digital cellular technology that uses spread-spectrum techniques.CDMA consistently provides better capacity for voice and data communications than other commercial mobile technologies, allowing more subscribers to connect at any given time, and it is the common platform on which 3G technologies are built.It doesn't have sim card.
The reason for more GSM usage is that the international Roaming of GSM is Very good and it has sim card,So you can easily swap your sim card when you need.For choosing your cell phone network,Go to a phone shop in town, tell them what you want, they can advise.Before getting your cell phone service provider,Make sure your phone is unlocked and accept that carrier,If it's restricted by other network carrier means,You will need to unlock your phone.For unlocking you can approach your current network provider or else try to buy an unlocking service from any one of the online unlocking service provider like Unlock-Zone.com and you can get free unlock instructions from MobileUnlockGuide.com I hope it will be very useful to you.