How can you learn PowerPoint?
GCFLearnFree.org has tutorials and online, instructor-led classes for PowerPoint. All for free. I don't know which version of PowerPoint you have but if you visit http://www.GCFLearnFree.org/PowerPoint, you can see the tutorials available for 2000 and up.
The online classes are based on the information you can find in the tutorials but it has that feedback that a lot of people like through having an instructor you can work with. The instructor led classes are only in PowerPoint 2007 and PowerPoint 2010. At the end of the online class, you'll receive a certificate of completion that you may also use for Continuing Education Units.
Complete disclosure: I work with GCFLearnFree.org but the program is supported by Goodwill and it is quality educational content for absolutely free. Check it out!