What other colours and patterns are in the room?
What is the surface you are decorating? Is the wall you are papering a bumpy old wall? It may need re-plastering or a textured wallpaper. Or are you painting wood? Do you need a primer?
Do you want the wallpaper/paint to be the main feature or do you want it to blend subtly with your scheme?
Think about the size of the room and ceilings. If the room is small you may want to keep to a small pattern and keep the colours light.