There are times when you have just one hand free to both hold the nail in place and hammer. One hand nailing is easier than it sounds. Simply wedge the nail between the hammer's claw with the point facing forward and the nail head resting on the hammer head base. Swing the hammer with the nail/claw facing forward. The force should give the nail a start into the surface. Remove the claw from the nail, and finish the job with the hammer head side.
It is always a temptation to take short cuts. You may choose to paint over wallpaper either because you don’t want to take the time to remove it, or perhaps the paper hides cracks and defects in the walls of your older home. Most professionals will suggest that you remove the paper because it may peel or the wall covering texture will be difficult to paint over.