Do you mean ''how do I send private messages to other people''? If that is what you meant, you click ''new messages'' which is between ''Trades'' and ''My posts'' then click the small white rectangular button that says in red text ''NEWPM *''. After you do all that your inbox will show up and you will have the ability to send private messages (also known as ''PM's'') to other people. To PM other people types their username(s) in the small white box below ''Compose message''. If you want to only write to one person, only write that one person's username in the small white box. If you want to send the same message for multiple people, add everyone's usernames in the small white box, but type their usernames in paragraphs, like this:
Username 1
Username 2
Username 3
otherwise your PM will be unable to send.