OR make a Unit, that looks like a sign, then do this
[trigger=My Trigger]MyTrigger
Events
Player - Player 1 (Red) Selects a unit
Conditions
(Unit-type of (Triggering unit)) Equal to Sign
Actions
Game - Display to (Player group((Triggering player))) the text: Your Text Here[/trigger]
Note: You have to place that Sign unit (The best place would be in front of something like Dungeons, Castles, Towns etc.)