None of the proposed "end of world" scenarios have any scientific basis.
The Maya did not predict the end of the world -- any more than Pope Gregory did when he developed our current calendar. The Maya "year" is called a "tun", and 20 of these is called a "katun". 20 katun is called a "baktun", about 4000 years, and on December 21, 2012, the 13th baktun will end, and on December 22, the 14th baktun will begin. This is just like December 31, 2000, the end of the 20th century and second millenium, and January 1, 2001, the beginning of the 21st century and third millenium. (I guess the Maya didn't wear closed-toe shoes, as they counted by 20s instead of 10s.)
Planetary alignments and galactic alignments? No special planetary alignment occurs this year. Every year at the time of the Winter Solstice, the same alignment occurs, and nothing has happened. Back in December, 2004, there was an alignment such that in the early morning, Mercury was near the Eastern horizon, Venus was a bit above that, then Mars, Jupiter and Saturn each higher than the previous across the zodiac. With all these planets on the same side of the Earth-Sun line, nothing happened.
There are no known comets or asteroids or other objects expected to impact the Earth in the future, but astronomers continue to search, observe and calculate orbits to monitor any objects which are likely to collide with Earth.
Same kind of discussion applies about any other "reason" for it.
Given all the above, I recommend the following:
1. Don't run up your credit cards and other debts expecting that you won't have to pay them back after December 2012.
2. Don't skimp on your studies because you thought that by the end of 2012 it won't matter!
3. Remember that "I read on the internet that the world was going to end, so I didn't buy any Christmas presents" will sound really lame on Christmas morning.