I have one red in color pimple about a eighth inch in diamiter on the head of my penis on top?

See remark. Is this recent ,or of some standing ? Not knowing your history ,did it follow sexual contact ? injury? does it itch ? Is it getting larger or smaller? Use an antibiotic ointment twice a day at the spot,avoid sexual contact..if it doesn't go away or increase in size,it would be advisable to see s skin specialist or a urologist. Wishing you the best in health.