Painted walls are rarely perfect - they're usually dirty, and often littered with screw holes you no longer require.
You will need
Step by step process
The key to a great finish is a clean surface, so wipe away the dirt and grime with Decorator's Cleaner. Made with concentrated sugar soap, add five full caps to a bucket of water (4.5 Lt) and use to wash walls with a cloth or sponge starting at the top and working down.
Screw holes you no longer need? Fill them fast with One Fill Polyfilla, an amazingly lightweight filler that's quick and easy to use. Dries in as little as 15 minutes, and you shouldn't need to sand it either.