Before painting, prime the walls. We are fans of using primers! Skipping this step, even on a repaint, can bring disastrous results to your paint job. In this case, using primer on the walls will serve as an added benefit in sealing in odors associated with years of cigarette smoking.
We would suggest a thorough cleaning of the walls using a solution of TSP (tri-sodium phosphate) which is available in any home center, hardware store or paint dealer. After cleaning the walls, apply a coat of oil-based primer. This will give you a clean wall surface to work from and ensure a terrific finished job for your newly painted walls, while sealing away any remaining cigarette smoke stains or nasty odors.