Here's a pretty detailed guide
Car Care Advice - How to safely clean and protect your engine bay , that should get you going. Generally, the engine should be cold and you'll need a degreaser of some sort, something like AutoGlym Engine and Machine Cleaner is pretty good and avaiable from Halfords, then a brushs (some poeple use paintbrushes, which can be cut down) and finally a sealant for the plastics, going again with AutoGlym Vinyl and Rubber care should do, or more specialist is 303 Aerospace protector.