There is no scientific evidence that anything will happen on Dec 21, 2012. All that we know is that is when the Mayan calendar ends.
There are wide-ranging opinions about what will occur, from nothing, to an evolution of consciousness, to disasters, to Biblical rapture.
The best advice I can personally give is "prepare for the worst, hope for the best, and enjoy life".
If you are seriously distraught about the negative possibilities - seek professional help.