To take a picture of a mountainside in the distance, with moonlight and without should i use M or P?
I get nothing on automatic.
What settings should i try to start with?
Do i change the ISO?
You're going to have a hard time with that shot on a t3i or even a t5i. On M you need to adjust your ISO and make sure your aperture is pretty wide if you're using moonlight. Put your camera on a tripod and make your shutter speed roughly 30 seconds. See what you get.
I just don't think those camera's have really high ISO capabilities. Much less clean like a 5D mkiii
Good luck!
I would take a different approach than the one recommended above. Set up your camera on a tripod, use like F8 - F11, a low ISO(better image quality once you dial it in), set it on B and start at 30 seconds. Keep in mind that the time doubles for every stop you increase in exposure, the shutter speeds will be 30 secs, 1min, 2min, 4min, etc. You will either need a cable shutter release or ML's long exposure settings. Keep light around you to a minimum as the camera might pick up the light on ground or trees in front of it.