If the walls or ceiling you wish to paint have been recently plastered then they will need to be completely dried out before any paint can be applied.
This could take several weeks depending on how warm the room is.
It is sensible to wait this long because if paint is applied to a plastered wall / ceiling that has not dried completely, any moisture will effectively be trapped and could result in mould growth and the paint may not ‘key’ to the wall / ceiling as it should, resulting in flaking and peeling.
New plaster should be sealed with a primer coat before you apply the top coat(s). There are manufactured sealants on the market but you could mix your own.
If you intend to paint the plastered wall / ceiling with emulsion, mix water with emulsion in a ratio of three or four parts to one and paint this on first to seal it.
Any ‘old’ plastered walls / ceilings will probably require a great deal of preparation in order to achieve a good paint finish.
All holes, cracks and knocks etc, will have to be filled and sanded smooth with an appropriate filler and sand paper.
Any walls /ceilings that have been painted previously should be cleaned using warm water and soap. If there are any stains, especially on the ceiling from nicotine etc, then try using sugar soap (available in powder form to mix with water or as ready to use wipes), this is more effective and recommended by most decorators.
Painting wall and ceilings to obtain the desired finish will probably require a couple of coats, dependant on the colour you are applying and any stains or marks that can’t be removed.
I always use a paint roller and tray for the majority of the painting and finish off corners and around woodwork with a suitable size brush.
Don’t overload the roller or brush with too much paint (after a short while you will be able to judge how much paint to load onto the roller or brush).
Cover any flooring or furnishings with a dustsheet that you don’t want speckled with the paint from the roller.
Give the paint time to dry before worrying that it looks a mixture of shades as sections of the wall / ceiling will dry quicker than others.
When you are finished, don’t forget to clean your paintbrush before the paint dries.