If you've never touched Photoshop before, start with the Help, which is included with the install. Not only it covers up all the aspects of the software, but also it links to official tutorial videos. And if the Adobe Help Air application is too heavy or constantly updating, the help contents are also available as 40+ MB PDF.
Learn Photoshop CS5 is a good landing page to start and all the video tutorials can be found from AdobeTV website.
If you're after those still convinced that official sources are good in quality, take a look at Adobe Photoshop CS5 Classroom in a Book. In my opinion the Classroom in a Book -series have good introductory examples with relatively easy hands-on exercises.
Remember that Photoshop is just a tool and as such pretty versatile: you could do ia. photo editing, vector graphics, animation, 3D, digital painting or websites with it. When you get the grasp of the basics of the software, there's still much to learn, but those sources should help you to get to started and—in time—you will think the software less and the design product more.