How do you private message on runescape?

Answer 1

This is only possible for people that are on your friend's list. Click on a person on your friend's list, and start typing the message.

There is more than one way to add somebody to your friend's list; the easiest way is, when the person types a public message, to right-click on that message (in the message area), and select the option to add that person to your friend's list.

Note that some players choose not to receive private message. In that case, the player will appear, in your friend's list, as "offline", even if you see him, and can talk to him in public chat.