96 Mercury Sable A/C problem

A/C worked intermitant. Now it will not cool at all. Yesterday the fan on the compresser started clattering when I shut the A/C back off. I'm pretty sure I need freon, but is that the only problem. My brother thought the clutch plate (I think that is what he called it) was either slipping or trying to freeze up. Would low freon cause this? Also, we could not find the port to charge it. Can anyone tell us how to find it A/C port? Thanks bunches!