okay, first, if he really cares about you, then he shouldn't care if you mess it up or not.
second, it's one kiss, not a make-out session, so it shouldn't last long and it's almost impossible to mess up. It's really simple, although it doesn't seem like it. All you do it tilt your head, close you eyes and let him do the rest, i was scared my first time too, everyone is, but it's okay and soon, you will be totally comfortable with it. i hope i helped