99942 Apophis is an asteroid, not a comet. In 2029, it will pass through a region of space that could potentially give it a slim chance of hitting the earth in 2036. Probability at the moment is 1 in 250,000. Even if such an impact were to occur with an object the size of Apophis (and irregular 270 meters or so), it might cause local devastation, but it would not be a doomsday, apocalypse or anything of that nature.
And I repeat, the probability of Apophis colliding with the Earth in 2036 is 1 in 250,000. Nothing to worry about.