You can examine the desk by clicking on it. The cursor changes to an eye when you mouse over an area you can inspect. This looks like an important object. You can click on it to pick it up.

The gear cursor shows places where you can use objects. The six-pointed star belongs here - take it from your inventory and place it here.

These sparkles indicate a hidden object scene. Click on them to enter the scene. On this screen, you must find all of the hidden objects on the list. Here is one of the objects - click to pick it up.