How to get the clearest action pictures on my Nikon d90?

I want the fastest shutter speed but when i set it at like 800 or more, i take the pic and then its like really dark. Like i can't even see the picture. What do i do! Please dumb it down for me! What is the best mode i can use for action pictures besides the "sports mode" because when i use that, my pics still come out a little blurry like where that most movement is.

When you use a fast shutter speed you need to set the camera's aperture to a bigger number to allow more light into the camera. You'll get better with practice. If you use the shutter priority setting on the camera it should usually calculate the aperture required for a clear photo, unlike full manual where you have to set both these values yourself.

To put simply, take the action pictures in bright daylight when the lighting condition will allow maximum amount of light into the lens while getting the fast shutter speed you want. Also, when you're taking a motion capture, you might want to "move in the direction" with the object while taking the picture so that the subject will stay sharp against the blurry motion background.

Also along with the others answers, practice panning. Move with your subject. This will keep your subject in focus and blurring the background.