Is my Nikon D3100 broken? How can I fix it?

When I look through the viewfinder, everything looks black and grainy.
I know it's not my lenses, because it looks the same through each lens.
When I take a photo, it comes up with a black "box" in front of it.
I can only see a sliver of the photo across the top! Help!

Added (1). Okay, so I fixed the black and grainy part.
When I take off my lens and try to take a picture, the bottom left mirror clip seems to have broken and the mirror pops out of place.
How can I fix this?

STOP! Don't do anything else! You need to either take the camera into your local camera shop (NOT bestbuy or walmart) or call Nikon (1-800-NIKON-UX… 1-800-645-6689) and make arrangements to send it to them.

I don't think you have the tools or skills to repair your camera. Has the camera been dropped?

You can't. Take it to nikon and see if they discover that you dropped it.

You don't fix a camera. You either replace it with a new one or bring it to the authorized service center for repairs. Ask the store you bought the camera from first to get an idea of what to do. If the cmaera is under a year old, warranty may still apply.

Camera dropped? The reflex mirror won't just come off like that.