Assuming drywall and waterbased pre-glued wallpaper;
Spritz or sponge the wall with water then wait for a few minutes. Use a scraper to gently separate the wallpaper from the wall. Be careful, as it is easy to dig into the drywall (not as much of a concern if you are replacing with different wallpaper...). Once all the paper is removed, use a sponge and water to clean the remaining glue off the wall.