Well if you mean in general put your message in the text box
below it and hit the "enter" key or click the Howwwwwl
button bellow the text box...

Now if you mean
certain people in the Chat box you can put there name the a
colon or @ symbol and it normally catches there attention on
your direct message to them.

Example
you
want to me to know something specifically like you want to
tell me you want to art trade... do this....

Silverwolf: Hey silver, wanna art trade?


Hope this helps,

Silver

You write a message and wait for a reply, to do so go to the
Shout Box, write your message, click 'Hooooowl' or press the
enter key.

below the shoutbox is a place where you put your message.
Then you jist click the button that says,'Hooooowl'. But you
need to be logged in to chat