First Theory is that the world will be hit by a planet with a highly elliptical orbit known as Nibiru/ Planet X.
1: If Nibiru was heading for Earth we would be able to see it by now.
2: No Telescopic evidence that this planet even exists in fact no evidence at all.
Second Theory is that the Earth, The Sun and The Center of the Galaxy will all be in line with each other and knock Earth out of it's orbit.
1. This is true, this will happen on December 21st 2012 ... But it also happens every year on December 21st
Third Theory is that the Sun's solar flares will reach there highest point and kill everyone.
1: The Sun Reaches it's maximum activity every 11 years!
2: Last time I checked I didn't die in 2001.
Fourth Theory is that the Mayan Calender ends on that day and something drastic will happen.
1: The Mayan Calender does not end on that day.
2: The Mayan Long count ends on that day which is every 5125 years I think just like 10 years is a decade for us.
3: As soon as it ends it will renew again, just like your calander ( if you have one!) ends on December 31st but then you renew it.
4: How did the Mayans predict something like that, they thought that the Sun would come up again if they stabbed someone in the heart! ( or was that the Aztecs? )
4: Fifth Theory is Earth's pole shift.
1: while this is real, it is a gradual process but maybe something our children's Children's Children's Children's Children's (Recurring) might have to deal with.
There you go! I hope I settled your nerves!