One of the simplest of all Black Magic is the Stickfast Hex. It is cast by simply waving your wand lightly clockwise and shouting "Colloshoo" while directly pointing your wand at the victim. It causes the foe to have their shoes stuck to the floor, so this should be used only for mild intimidation or a diversion for another spell more powerful than this, but should not be used too much for the distraction, as more advanced wizards and witches cast a spell at you right when you cast the hex at them.

Another one is the Cruciatus Curse, cast by shouting "Crucio" with no wand movement, but just with your wand directly at the victim. It is a torturing curse mostly used to force something out of the victim, though it works excellently in duels and battles as well. 

The last curse is Sectumsempra, invented by a former Death Eater and Hogwarts Professor, Severus Snape, in his childhood. It is cast merely by the incantation "Sectumsempra" with your wand pointed at the victim - directly at the victim - and use it to let the enemy feel pain as if they were slashed by a sword, also causing plenty of blood. Many people are known to use this curse instead of using the Cruciatus Curse so they stay safe from the Ministry of Magic.