Okay. It sounds like you could route the wire either direction from the electrical panel, UP into the attic, or DOWN into the crawl space. You will need the following items:
50' of 12-2 w/ground romex (which will have a yellow outer covering)
a single pole 20 amp breaker that matches the brand of your electrical panel (there may already be a spare breaker in the panel)
a cut-in electrical box (single gang, meaning it will only contain 1 outlet
a standard outlet that matches the configuration of the prongs on the plug for the microwave
a faceplate for the outlet
You will need to route the new wire into the breaker panel from above or below the panel, through the inside of the wall. The white wire connects to the neutral bar, where all of the other white wires are connected. The black wire connects to the new breaker. The bare wire connects to the ground bar, where all of the other bare wires are connected.
At the other end, you have to cut a hole in the drywall to accomodate the new cut-in outlet box. Before installing the box, you have to get the wire to the hole you cut for the box, by chasing it through the wall from above or below. You will likely have to drill a hole in the top plate of the wall (from the attic) above the new outlet, if you choose to route the new wire through the attic.
Once the box is installed in the hole you cut behind the microwave, and the new wire is in the box, strip off the outer jacket, and connect the black wire to the gold screw, the white wire to the silver screw, and the bare wire to the green screw on the new outlet. You need a romex connector
for the electrical panel, to put the new wire through. Your local home improvement store will know what all of the items are, so they can sell you the proper parts. If you need to see pictures of any of the parts, let me know.
It may not be a bad idea to print out this answer, and highlight the parts list, and take it with you to the store.