Any idea what's wrong with my camera?

I own a Nikon D3100 with a does lens of 18-55.

Since yesterday, I was having problems with my camera. I can't really see through the view finder and when I take the picture they all come out like this.

With a bit of the image at the top?

I'm taking it to a shop / best buy tomorrow. Any idea what's wrong with it so at least know what to tell them?

I would suggest taking it to an actual camera shop and not BestBuy. A camera shop would have more experience dealing with DSLR issues than the "certified geeksquad" employees.

Go to http://www.yellowpages.com type in camera and your city, state (I avoid Wolf and Ritz cameras though)

This happened to mine too, and then it just turned completely white, and nothing would work.

You don't need to know what's wrong with the camera. What's important is that it doesn't work. Just show what you've got and demand for a replacement if the warranty still covers it. If not, you'll have to wait for it to be repaired. Very important. Bring the receipt. Return only to the exact store/branch you bought it from.