The spark plugs on a 1997 Ford F-150 4.6L engine are located on the top of the engine, between the valve covers. To access them, you'll need to remove the ignition coil packs. There's one coil pack per two cylinders (for a total of three coil packs for the V8). The spark plugs are directly under the coil packs.
Because there's no single easily-described location, and there are multiple spark plugs, a repair manual specific to that year and engine is highly recommended to pinpoint each plug's location accurately and safely. A visual diagram in such a manual will be invaluable.
