wallpaper over bare, unprimed sheetrock is a disaster.
you can remove it mostly, but will lose a lot of surface paper from the rock at the same time, or there are 2 other options.
first - and easiest, as long as the wallpaper isn't bubbled or peeling, paint it with a laquer based primer like KILZ. (there are several good ones out there, that's just the first to come to mind) do not use the latex based version - it's got enough water in it to cause adhesion problems with the wallpaper. Once the paper is primed, you can give it a second coat of good acrylic latex primer, then texturize (splattercoat and knock down, I suspect as that's pretty common). prime your texture coat with PVA drywall primer, then prime and paint with acrylic latex.
second - not quite as easy or inexpensive, but better overall results, put up a layer of 1/4" or 3/8" rock over the wallpaper, and treat it like a new room - mud and tape your seams, do your texture, then PVA drywall primer and paint.
PVA drywall primer is not the same thing as regular primer - it's designed specifically to seal fresh sheetrock and/or mud. I've seen commercial jobs where they decided regular primer would be good enough and ended up needing 2 coats of primer and 3 topcoats before it stopped soaking in and showing "flat" spots.
personal preference note - I never use flat paint. It shows dirt, is harder to clean, stains easy, soaks up humidity like crazy, and is not as durable a surface. Eggshell or satin finishes seem to be the best compromise between easy care and appearance. (gloss finishes are the most durable and easy to care for, but seem to magnify every imperfection in a surface)
sorry the answer is so long, hope it helps.