First, you must create a new window with a static field where you like to appear the subtitle. See this--->
http://res.dead-code.org/doku.php/wmebook:ch12Second in the scene_init.script of the scene where you like to put the subtitle, call the window, and get the static field. Then set the text of the field with your subtitle.
the code in scene_init.script its something like this:
Game.Interactive=false;
var subtitlewindow = Game.LoadWindow("path and name of the window");
var subtitle = subtitlewindow.GetControl("name of static control of window");
subtitle.Text="Welcome to Peradeniya Park";
subtitlewindow.Visible=true;
Sleep(5000); //time that the window its visible
subtitlewindow.Visible=false;
Game.UnloadObject("subtitlewindow"); // destroy the window from memory
Game.Interactive=true;
this is the basic for this. you put this code in all scenes changing the text of the static control from the subtitle window.
note: you can create a function type showSubtitle(String text); and you can call this function in all scenes_inits. But this is more advanced, read the wme book to learn this.