How do I build a wooden ramp for a metal 10X8 foot shed with a 9 inch drop to ground and an opening of about 4 feet?

This will be hard to answer because you did not specify what will be stored and the dimensions of the building entrance.

Building a ramp requires a frame to be built so that some kind of decking material can be placed on top of it to provide a flat surface for the storage object to ride on.

The easiest way that I can think of is to get some joist hangers that allow joists to be placed on a downward angle. Install the joist hangers to the building's framing member located below the entrance. Install joists in the joist hangers and angle them down to the ground. Cut out the bottom of the joists so they allow the joist to sit on the ground nicely. Basically, you will make the cut so that the joist comes to a point where it meets the ground. Now place decking material on top of the joists (plywood, 5/4 decking boards) making sure the joists are all parallel and square. Now drive fasteners along the decking material to attach it securely to the joists.

Check your local building codes to see if permits, surveys, or inspections are needed. Code enforcement officers are your friends when you need to build things to accommodate heavy loads or special situations. (Just make sure you ask them questions before you start building stuff)