Simple button using Unity 3D 4.5, 2D game

How do I make a simple button in a 2D game? Using Unity 3D 4.5. I already have a button.png image file, I have added it as a sprite/gameobject. Where do I get from here? I am targeting iOS, however I guess it shouldn’t matter.

(the next step will be to find out how to load another scene, replacing the first one, on the button tap)

You can use GUI.Button for buttons.
Unity can create 2D or 3D games, but GUI will work for either of them.

I’m sorry but I just went with Unity 4.6, and 2D stuff is just so much more simple there. No reason to use 4.5 at this point since 4.6 even was released yesterday.