You must do good prep work. Fill holes with puddy, sand smooth with sandpaper on a block. wipe walls with TSP, remove wallplates and tape off baseboards, etc.
Good advice is when trimming with a brush prior to rollering, don't carry the full gallon of paint around!!! Pour a small amount into a clean container like a coffey (sp?) container. Accidents do happen.
Do your prep work, buy a GOOD top of the line paint, one or two coats is your preference.
Also be sure to pick a good color. I perfer "Medici Ivory" off-white from SW. Not too light & not too dark with a hint of yellow and red...