Circle collision is easiest; you see if the distance between the center of both circles is less than the average of their diameters.
Check out the CircleCollision Example.

i want to control the ball with mouseX,Y though, how would i go about that?

Which ball? What do you have so far?
The mouse location is specified with mouseX and mouseY, so if you call ellipse(mouseX, mouseY, 50, 50); it will draw an ellipse at the mouse position with a diameter of 50.