Facebook private message

hi i have tried to find my question in facebook help and somewhat does not give me the answer. i have relatives in other countries all over the world, and i would like to private message to one person exclusive at one message.
i do this from either from my computer or my phone ( android).
Also i found on my phone, i had to download messenger, and when i tried to send a message i was
charged by my phone carrier extra for doing this for overseas text.
so i would like to be able to do this for free from phone facebook and computer facebook without being
charged , more so be able to do this without messenger on my phone and uninstall it as it takes over my
phone for all messages, which I don't like.