Smooth out imperfections by applying drywall compound to any dings and dents. feather in patches by sanding them starting at the outside and sanding toward the center of the patch.
Setup your paint by pouring a couple of gallons of paint into a five gallon bucket. Hang a roller grid ( available at most hardware and paint stores) on the rim of the five gallon bucket. Paint trays are unhandy and have a way of finding your feet. Attach the roller frame to an extension pole, a two or three foot roller pole will save time and your back.
Apply paint to the roller. Do not dunk or plunge it into the paint. Daub it into the paint, roll it up the grid in order to rotate the cover then daub it back into the paint until the entire cover is loaded with paint.
Paint over all the patched areas with a light roller coat of the paint. this will lessen the likelihood of dull spots in the finished wall
With a brush paint ( "cut in") a three inch line into the inside corners, above the baseboard, around woodwork trim, and the top of the wall (at the ceiling).
Note: I suggest that you "cut in" the entire room before you start to roll (this may go against advice you have received from others). Start rolling back at the first wall you "cut in" or allow the paint to dry to touch before you start to roll.
Llghten up on the roller as you roll into the "cut in " paint.