This is worth a read. Also I have a couple of tricks (tips) I teach my photography students. When using a crop sensor D-SLR such as a Nikon 3100, if the zoom is set to 18mm and the aperture to f8 focus on an object at least 2.5 meters away from the camera and everything from half way back to the camera and the mountains would be in focus. Use f11 if you want a safety margin. With a full frame you get the same effect using a 28mm lens but need f11 or f16 if you want a safety margin. Now once you start using telephoto lens and 55 mm is a short telephoto lens on your Nikon 3100 you can’t get objects close to the camera and mountains razor sharp. A DOF guide on your smartphone can help with understanding this.
Good luck continuing to learn and improve your photography. We all still do even after many years. But avoid wide open apertures for landscapes. Wide open apertures are great with portraits but not so good with landscapes in my experience.