The spark plugs are located in the center of each cylinder, in between the 4 valves for each cylinder. There is a long port that runs through the valve cover and down to where the plugs are screwed in. You can see these ports in the attached pics.
You don't want to just willy-nilly mess with your plugs. The service interval for replacing them is 16,000 miles (24,000 kilometers), which happens to be the same for checking/adjusting the valves. This is no accident, as you need to remove a whole bunch of stuff to get down in there.:smile10
Do the valves and the plugs at the same time if you can.