settings for a Nikon D90 for skateboarding photography?

The main question- Do you want blurred action shots or frozen action?

If you want to pan and create a blurred background, use 1/125 or 1/60 of a second. ISO will be 200, aperture dependent on the available light.

If you want frozen action, then use 1/500 of a second or faster, ISO 200, aperture dependent on light.

The majority of skateboard shots are wide angle and fish eye lenses. So go with your widest lens