what do i need to do to my iphone to use it overseas?

If you do not want large roaming fees you might disable cellular data (or mobile date as it is sometimes called) and seek out free wifi if possible. If you have imessage set up and active do not turn it off when overseas as activation of imessage requires the ability to send SMS and you may not have that available when away. Make sure your phone is unlocked so that if you want to use a local SIM overseas you will be able to. If you can stand internation roaming costs then enable that with your carrier before you go away. Consider turning off data roaming

I make a point of enabling dail assist in settings/phone and entering all the numbers in contacts in international format, ie +(country code) (area code) (phone number). Make sure you eliminate leading zeros from area codes and before mobile (cell) numbers.

What country you in?