If the old is well-adhered & very clean, then buy a paste made for going over old wallpaper.
If the seams are going to telescope through the new wallpaper, then block sand them down & prime with a stain blocking primer.
Unless you install something like bead board over the walls, the wallpaper needs to come down first. You will regret it later on if you don't.
Are you wanting to paint over it? Here is a tutorial for Benjamin Moore.