There're two main tools of PowerPoint, we'll be using here to create icons. The first is the ability to edit the points of a shape. The novelty in PowerPoint 2010 is that you can right-click any shape now (not only free-form) and edit its points. The second tool is completely new to PowerPoint 2010 "combine shapes".
Here're video walkthrough: The first shows how to customize the ribbon to make sure you've the tools you need available:
Second shows how to create an icon of a light bulb – this's a simple image that can represent an idea, brainstorming or a concept:
third video shows how to create an icon based on a real world object, primarily in order to maintain real-world proportions: