To get rid of all the horrible people in the world that mean nothing but harm. Like the abusers, sexual predators, gangbangers, robbers, killers, etc. Get rid of them all.
If it could only be one...I guess I would change the homeless situation. There's too many people/families living on the streets and there's no reason for it to be happening in this country in this day and age. But this is a tough question because I have a few others, so it was very hard to choose just one.
If i could change one thing about the world. I would magically create a place where all children grow up happy, well loved, taken care of and taught right from wrong. I would ensure a loving, well-rounded childhood, I belive this is the most important to creating a more peaceful world.