I've decided I want to build an open air shed with corrugated fiberglass
panels for a roof, and 4x4s for uprights, in my yard for tractor, and yard
vehicle parking. The shed would be 20' wide, by 8' deep, by 8' height.

I've done carpentry, but have no experience or knowledge of how to layout
and design my project.

The main thing I need to know is how many stringers, running in what
directions, I'll need to support the roof panels. What kinds of fasteners do
these panels need?

Where do I start on this?

I'd start by looking at these two sites. The first is good for
inspiration on smaller sheds and aesthetics, and the second better for
what I think you're looking for.

www.readersheds.co.uk/

http://www.ag.ndsu.nodak.edu/abeng/plans/

Barry
www.woodworking.wizkids.co.uk/