The walls need to be cleaned, to get any dirt and grime off of it so the new material will adhere better. If you have a high gloss on the existing wall, you can use TSP, to prep the surface. Depending on how dirty the walls are will depend on what you use to clean with. Usually a mild soap and warm water will do the trick, but be sure and rinse. Then after the washing, some people will suggest that you prime the surface to make everything even up for the new texture, thats your call. If there is no texture on the existing wall, then you may want to scuff it with sand paper, (80 grit) to help the new texture stick.
I use beadex taping mud for my orange peel textures. The glue in the mud will help give it tooth.