Dark Nikon D90 aperture problem

I'm just starting out with playing on the A mode of my Nikon D90, problem is, all my images are coming out really dark, and i don't know why, also, i can't seem to change the aperture, i move the swirly dial on the front and look through the view finder, but nothign happens:/

Isn't "A" automatic? If your images are dark then there must be something wrongly configured.
Your camera is an SLR. When you adjust the aperture nothing happens until the shutter is triggered when the aperture closes to the selected position and the "shutter" opens to take the picture. Somewhere on your camera there's going to be a button that will allow you to close the aperture to preview the depth of field, etc.before taking the picture. You will have to find that in the manual but at least now you know that you are looking for the preview button (or whatever they choose to call it.)

Yes, keep turning the front dial. If the f stop setting is at its max or min, turn the dial in the opposite direction. Watch carefully to see the f stop change in the viewfinder window. If you're pointing the camera at a predominantly bright scene with few or no shadows/dark areas, the camera will underexpose the scene. You might also check to see if you have accidentally set exposure compensation with the +/- button. (look these up in the manual if you don't know how to check)

"A" is not Automatic, on Nikon camera's it's Aperture Priority. When you make adjustments to the aperture, the camera doesn't actually physically close or open the aperture until you press the shutter release. If your image are turning out dark, it means you may have an EV adjustment set. If you've just been swirling dials then you likely have accidentally told the camera to underexpose by two stops or something. Make sure the EV on your screen says +0 or -0. As a test, go outside on a sunny day and try shooting at a wide aperture (zoom all the way out and try f/3.5 for instance). There should be plenty of light.