How can I do this simple script?

So.. I've made a script that goes like:

function Regen() game.Workspace.Wizard.Humanoid.Health = 100 end

???? <--

How do I make it so that when the Wizard's health reaches 25, a Gui pops up saying "Wizard Used Heal" and the health goes back up?

(It's a script, inside of Workspace. So.. should I do a Local Script?)