hey how do I message people? sorry really new to this game still
the reason I want to know is because I got messaged from some guy and he asked me to join him and his legion and I want to join but I dont know how to message him back

If you can find their ship, you can post on his comm. Otherwise youll have to look the legion up and request to join and hopefully that will trigger him to ask you again and you can reply on your comm.

how do I find a ship? still very new to the game

The same way you found it in the first place, if they messaged you then try responding on your comm, they might be watching for a reply. Otherwise request to join them.