Auto-exposure is almost always bad. Use manual unless pressed for time.
Pre-Programmed mode (portrait, landscape, night mode) should almost never be used.
Aperture and shutter priority are better than auto but worse than manual but for beginners it is probably the best bet once you understand the basics of exposure but are not yet ready to do the match in your head for manual exposure.
Use spot metering where possible. All cameras default to "evaluative" or "average" metering. This takes an average of light reflected from all objects in a composition and you do not want that. You want to almost always expose for a given point in the frame -- a face, a building, a mountain, sky and so on. That's called "keytone" exposure by the way.
Given an option between under exposure and over exposure always go for under exposure
