Semi gloss or satin is easier to keep clean. You can clean the walls with tsp before you paint, mostly where greasy hands marked up the wall. Home depot has a newer tsp type product that doesn't need to be washed off. It's a pink liquid in a clear bottle. If you dont use tsp at least spray some kitchen degreaser around the light switches n doorways to get the schmutz off. If you do any drywall work the raw drywall and drywall mudd needs to be primed with a primer before painting. Even if the paint says primer n paint in one. I was told to always prime first. Yeah go right over the old flat paint with whatever you choose no need to prime over paint unless its smoke stained or something.