Communicating from another country? Tmobile ?

Ok so I'm going to leave to mexico in a month and is there a way I can communicate with my friends for free or without extra charge? I will be able to connect to a wifi for free over there. I have a tmobile sidekick4g that can wifi call stuff. Will it be free? I know it will deplete my minutes but I don't care about that I just don't want any extra charges. Or is there an app I can text or communicate for free. Sidekick 4g is android so yeah. Any help people???