In this day and age there are so many different paints to choose from that anybody that only slightly know what to choose can walk away with a headache. Paint falls into two main categories, oil based and water based. Stains and also varnishes also fall in these same categories.
Oil based paints have a gloss, semi gloss, or matte finish. It wears well and it provides an easy clean surface. Oil paint is good to use on doors, windows, or anywhere that needs a protection finish. You can use oil based paint on wall and ceiling if you require a tough washable surface, like oil paints usually. It is never a good idea to paint over top of wallpaper because it makes it harder to remove from the wall.
Water based paint is much easier to work with this is because the brushes and all painting tools can be cleaned using only water, which is similar to paint cans to most people. It is the choice paint for walls and also for ceilings and for textured surfaces outside.
On any new surface it is a good idea to use an undercoat or a primer. By doing this you will prevent any paint from being absorbed. Also by doing this it gives the paint something to stick to. The primer the you choose should be manufactured by the same company. Also you can use a water based primer for an oil based top coat but it is not a great idea to use a oil based primer for a water based paint. The number of coats the you will need depends on how porous the surface is, and the thickness of the paint you are applying, like painting tools usually. The overall quality of the paint has a big factor too.