Take it from one who knows............... the hard way................. go to Lowes, Home Depot, or any hardware store and get TSP. Mix it up according to either directions or the advice in the store.
What a horror story I lived trying to get paper off a wall that was never prepped, never painted. TSP SAVED ME. TSP stands for trisodium phosphate. It is actually an industrial cleaner (and also a food additive) and it cuts through grease and grim like crazy. I swear by this stuff and have also used it to "degloss" painted areas and then don't have to sand. Anyway,............ back to the wallpaper story............
Use the diluted TSP powder double strength and soak the paper with a sponge or use a spray bottle. Just please remember to wear gloves and make certain that it doesn't get in the eyes. This is serious stuff.