You'd want to remove the crown of the sparkplug first, it's threaded. The crown is the part that is above the porcelain when it's screwed into the head. If you see threads already - then there is no crown, and you would want to look into the boot and insure that there is a clip in there to receive the threads. To see if it's intact in the boot, you are looking for a bronzish cylinder with a hole, and two silver wires parallel to each other. If those silver wires are not there, it will not receive, though my money is on that you didn't remove the crown.