Clean it really well, I would put a couple of coats of Kilz or a product like it to keep the dark color from bleeding through the lighter paint color, follow the directions for wait times between coats, then paint it the color you want.
Wash the wall, prime with Kilz primer, paint your color.
Prime with Zinsser first. One coat should do it. I just went from a deep red with no problem.
Most quality paints have a primer mixed in so you should be okay to just paint.
Although most paints come with a primer in it, I've noticed by using a separate primer I get the best results. The paint color comes out more even!