Nikon D7000 Autofocus in live view, out of focus?

I've just bought a Nikon D7000 and when I go to auto focus in Live view it will go out of focus… If i press the shutter button half way for a second time it will go even further out of focus… And then eventually it stops focusing what so ever… Any ideas? All i have on the screen is a little rectangle with is either red or green… Help please!

You typically focus by holding the button until the focus has found its mark, not just by half-pressing shortly. Just making sure.

Furthermore, for the autofocus to work it must have something contrasty to work on. Try focusing on a tree or something like that, not a flat white wall. Are you already doing this? Some more details would be helpful.

Does the normal autofocus in the viewfinder work?

Does Autofocus work when the picture is taken?

Auto-focus works entirely different in live view mode, since the proper af module is attached to the mirror which flaps up when switching into live view. It works more like the autofocus system on point and shoot cameras, constantly focusing back and forth until the highest contrast is reached. Therefore you can't focus on a plain white wall, as Jens pointed out. You should be able to move this little rectangle and set your focus point, then press the shutter button half way and wait until it's in focus. It may take a while, so don't be overly impatient. A green rectangle indicates that the camera thinks focusing is completed and correct (But it does not necessarily mean it is), red rectangle means the camera couldn't focus properly, usually because of a lack of contrast.

Stop taking pictures in the dark. Go to the light!