Scuff sand the surface you wish to paint by quickly rubbing over it in circles with a piece of sandpaper. A sanding screen attached to the end of a pole will also work and will make it easier to reach high areas of the surface. Don't spend too much time on this step, but make sure you cover every part of the surface.
Clean off any dust that has accumulated on the surface after sanding, using a dry cloth. You can also use a broom or a dry Swiffer duster to reach high areas.
Prime the surface if you plan to paint with a bright, solid-colored paint. Light colors, pastels, whites and off-whites should not require the use of a primer.
Paint the surface as you normally would after the primer has fully dried.