iMessage to a person in a different country

Hi, would you let me know how to use iMessage with someone in a different country?

Do I need to use a country code to use iMessage and drop "0" just like we make a phone call?

(iMessage to Japanese iPhone). Just like Facetime, it is free? I experienced in the past that iMessage doesn't go through even though I was able to use Facetime....is it something to do with a country code?