prep and sanding

the most important part of the painting project is of course the prep,
the better the surface prep, the better the finish will be,
you want to make sure you patch all holes, cracks, voids and seams, with either,caulk, spackle, compound, or putty( which is typically used on trim only,) once you have patched all those area, you'll need to let it dry, and then sand to a smooth finish, after you must apply a primer on all your repaired areas,
doing so will prevent flashing, which is caused when your patches bleed thru because they havent been primerd, always apply a primer it will hide all your patch work, before you paint,and make you walls shine a uniform color.