Unity 3d help GUI and curser lock like myst game?

Hi I'm trying to develop a game by my self and it's so far doing good but I made a recent change in how you advance in levels instead of just running onto a platform I wanted a GUI but I'm not sure how to make it so it only appears when you click on a object and then click say a picture to advance in levels kind of like the myst games and a cursor lock script would be nice I'm using java script as my code so please no other coding languages and thx