Scrape off all of the old flaking bits of paint and any other loose bits of plaster etc... Open up any cracks by using the corner of the scraper like a knife, if you just fill these, they will re-open again sooner rather than later. Get some sugar soap for the fire damage and wash it off. Also wash off any other dirty areas. Go around and fill any cracks and holes etc. When filler is dry, sand these with a light sandpaper ( Circa 180 grade sandpaper). After sanding, make sure to dust away any dust as this will remain on the wall, very important unless u want grit all over your new paint job. You mentioned water stains, is it in a damp/moist area? If so, use a paint called steracryl for the ceiling, it is a mould inhibiting paint, good for areas with lots of moisture. You can use some cheap paint if you wish for the walls to prep them. Get a 2 or 3 inches brush for cutting in. Paint ceiing first, when painting ceiling, go around and cut-in first making sure to come down on the walls aswell. Then paint the walls after, that way you wont see 3 colours (i.e. the ceiling, walls & old colour in areas you didnt cut in properly. Give the ceilings 2 coats. Give a prep coat, then 2 coats to the walls, see how it looks and if needs be add another coat.