If you haven't already, you'll need to create an english.lang file; this will contain the information for your messages. Read up on that
here.
But it will look something like this:
<CATEGORY Name="Messages">
<Entry Name="Message01">Hello world</Entry>
</CATEGORY>
Now in your map script file, use the following to call the message (obviously, this has to be called using a function, how you do this is up to you):
SetMessage("Messages", " Message01", 0.0f);
The float value (0.0f) determines how long the message is displayed.