Q:

How was Mojo Jojo created?

In the PowerPuff girls we know the girls were created from Sugar, and Spice, and everything Nice, plus the secret ingredient, chemical X.

What caused Mojo Jojo to turn from a normal monkey, into a super evil genius?

A:

Physically, a Chemical X explosion; psychologically, living rough and uncared for.
From the Wikia page (emphasis mine):

During the event of the creation of the Powerpuff Girls he pushed the Professor towards a container of Chemical X, which caused an explosion, mutating his simian DNA, turning his skin green and causing his intelligence and brain to enlarge, growing out of his skull. It also gave him new emotions like envy and inferiority.
[...]
After Blossom, Bubbles and Buttercup were created, the Professor lost interest and/or attention of Mojo's existence and forgot about him. With newfound intelligence, he donned a tattered overcoat with a partially shredded scarf and a paper bag to cover his brain and ran away, setting his home up in an alley. Spending time on the street by himself with no one to care about him almost immediately turned him cold and emotionless providing a motive as to why he would care to destroy Townsville and conquer the world.

References: the episode Mr Mojo's Rising and the film The Powerpuff Girls Movie.

A:

As seen in The Powerpuff Girls Movie and the episode "Mr. Mojo's
  Rising", Mojo was originally Jojo, the Professor's hyperactive lab
  assistant who always broke everything. During the event of the
  creation of the Powerpuff Girls he pushed the Professor towards a
  container of Chemical X, which caused an explosion, mutating his
  simian DNA, turning his skin green and causing his intelligence and
  brain to enlarge, growing out of his skull. It also gave him new
  emotions like envy and inferiority.

The Powerpuff Girls Wiki

