Wendy wants to clean years of nicotine off of her walls and repaint.
First, the best cleaner for any paint preparation is TSP, Tri-Sodium-Phosphate. There are now some more environmentally friendly "TSP Substitutes" available on the market as well. If no stain is left, use any paint you want. If there is a chance that there is still a bit of stain on the wall, then you should start with a stain block primer.
There are three kinds of stain block primers:
The oil-based stain blocks are better.
The shellac-based stain blocks are the best.
Any time you have tar or solvent based stains (like cigarettes or felt markers) you need to choose a white shellac primer like the Zinsser brand or an aluminum spray paint. Otherwise, with time the chemicals in the stains will work their way right through layers of new paint.
There are many types of primers available, many of them quite specialized like the Super Adherent primers" that we use to paint glossy surfaces like tiles. Stain block primers are either not very good paint primers or very expensive primers but they do block stains better than most regular primers.
Surprisingly, for felt markers, spray painting metalic aluminum paint in such a way as it looks like graffiti itself, actually works as long as there is no more felt marker visible. A regular white paint primer will easily and totally cover the aluminum work of art and an even white wall is ready to paint. Nothing will bleed through the aluminum layer. Aluminum paint can even be used to prime the black plastic cement used for roofing joints. It reflects the UV light of the sun and makes the patched joint last longer.