How do I do in Unreal Script a Text Messege that apear on the game screen?

I suggest you'd start reading some tutorials first. www.unrealscript.com has some good ones, also the tech page has great documentation about UnrealScript.

The function to make a message appear on the screen for all players is BroadCastMessage. This will place the tekst moo appear on the upper left part of the screen.