What he said about the manual. But to your two questions:
Set the shutter speed dial to "A" (with the AE prism on, and one of the "Ax" focusing screens installed, like Focusing Screen AE) and the camera becomes an aperture priority automatic camera. The readout in the viewfinder changes to show you the shutter speed it will be selecting, at the bottom. And if you have a "new FD" lens you can see the aperture you've set, read directly off the lens. A little known fact is that you can do this even with the standard eye-level Prism, or the Speed Finder, or whatever. All the AE prism gets you is the convenience of the readout telling you what shutter speed it is selecting.
If you have a motor drive or winder attached (this does not work without one, which is frustrating!), then setting the aperture ring to "A" (or the little green circle on older lenses) turns it into a shutter-priority automatic camera, and you set the shutter speed, while the needle in the viewfinder tells you what aperture it's going to be selecting for you.
You can't set both modes active and get "P" mode, sorry... I think its behavior in that case is somewhat undefined.
The little R post is the rewind button, like is found on the bottom plate on most other cameras. Use it for rewinding the film, taking double exposures, etc.
Duncan
