A humongously large asteroid will collide with Earth, killing nearly everyone with the shock waves immediately.
These shock waves will also set of the worlds stockpiles of nuclear and chemical weapons which will take care of the rest with the fallout within an hour.
The Earth will never be populated again with life forms, as it will be on a one way trip out of orbit to the Sun to finish off the last surviving bacteria that will end up as crispy critters.
Ok lets crank this up a gear or two for fun what would you do if there was a zombie apocalypse, think the walking dead type scenario, personally I would raid pharmacies and hospitals ASAP then hunt for food.