Measure the surfaces to be painted so the paint shop can work out how much paint you need.
Remove wall sockets and switches so you don't have to mask them. But remember to cut the power first!
Move out as much furniture as possible to make room. Beds and heavy furniture can be covered with furniture plastic if they are difficult to move.
Pull out wardrobes that are not wall-mounted, so you can paint behind them.
Test paint your chosen colour on a piece of cardboard and move it around the room. Then you can see how the colour is perceived in different lights.