OnMouseEnter() over environment GameObject during the game

Hello and thanks in advance,

My question:

I am trying to do a kids game. My target is this: when I point with the mouse the tools I find (pencil, buckect, etc...) around the environment, they should blink (the image should roll over, or whatever) or maybe the mouse pointer would change its shape. In other words I should know I can do something with them. (The game must be 3D)

The problem: I am totally new and I need to know where should I find, in the reference manual I didn't find anything. I know how to make a menu and all this within the menu, but not during the game.

Thank you very much.