It sounds like you might need to pick up a basic repair manual/guide. Although it is not exactly a repair manual, there is a great book called Cheap Outboards that walks you through some of the basics, regardless of model. The impeller is located in the lower unit; the water pump housing is just above the gearcase. There are several bolts that you have to remove to drop the the lower unit to get access to the water pump housing. Inside, you will find the impeller and key.
You must first remove the propeller and rubber clutch to get to it. The impeller is on the propeller shaft behind the brass plate you will see after you remove the propeller and clutch.
