After reading some of the other replies, a word of caution is in order. It is entirely possible (actually, a certainty in my case) that the substrate is plaster over wood lath, and the wallpaper was applied to hide the cracks in the plaster. To prepare this for paint requires sanding below the surface along the cracks, then applying joint compound and tape. Then, sanding and (finally! *whew*) paint. Some will say it would be easier to just slam up another layer of drywall, mud, tape and paint, and they would be right at least some of the time. Some of us prefer to preserve.
Like, especially in rooms with lovingly restored tongue and groove ceilings.
Beaten into submission on that score by SWMBO, but she was right.