Francine, how long has it been since the walls were painted? If its been 5-6 years or more, then prime it with zinsser's SHIELDZ. This is both a primer and a sizing. Otherwise, just put on sizing.
The sizing (and Shieldz) helps you do a better job hanging the paper. The sizing helps the paste adhere to the walls. You especially want this on the seams, where most people squeeze out the paste causint the seams to curl. Both products minimze stretching of the paper. Stretching happens when trying to position the paper and when smoothing it out. Stretched paper will shrink, exposing the seams.