How do I stop my Android phone using data when abroad?

Hi!
I am planning to go for vacation or out of the country this coming May. I am just thinking if you can help me with my concern. Some of my friends advised me, that if I am going to use my Android phone outside country, make it sure that it is not using data. I am just wondering why I need to do that. How helpful for me to do that? Can you provide me step by step resolution to do this? Can you provide me alternative ways on how I can easily do this in my phone?
Please help!
Thank you!
