Download an instant messaging program that you know the someone in question uses. Then add them as a contact. When they are online, most IM programs will put them to the top of the listing, and show an indicator icon that has a green colour or other positive indication.. . While they are online, right click over top of their contact line, and choose whatever menu item is the closest analogue to "Send Message." Type your message, and then click whatever button is closest to being labeled "Send."