Muscle up (along with Human Flag) is one of the most difficult exercises of calisthenics. Works almost on all your upper body and only one rep needs months of hard work to accomplish.
It is not the exercise along which is difficult, but difficult is and the grabbing, too. In the beginning you will find this exercise strange. Begin by doing the classic Pull-up and when your head is above the bar, then pull your body, so you can stand on your hands.
Watch the video below.