1. There are _no_ planetary alignments in 2012. I know because I've checked for them myself in Starry Night software.
2. There is no "dark rift" at the centre of the Milky Way. There is a black hole, but it has existed for billions of years and is too far away to have any effect on Earth. The Sun does not align with this, but is 6 degrees off, and does this _every_ year, not just in 2012. Again, see Starry Night.
3. Polar shifts take 500 years, not one year, and there is absolutely no evidence that one is about to happen.
4. The current cycle of the Mayan calendar ends on 2012 December 21, but they made _no_ prophesies of what would happen then. Presumably, if they were still around, they'd just start a new cycle. That's the way _all_ calendars work.
The world has existed for over four billion years. Is it reasonable to expect that it will come to an end in less than four years? And all because of a Mayan calendar? Use some common sense.
The most interesting astronomical events in 2012 will be an annular eclipse of the Sun on 2012 May 20, a total eclipse of the Sun on 2012 Nov 13, and a transit of the planet Venus across the face of the Sun on 2012 Jun 06.
Scientists don't expect anything out of the ordinary to happen in the year 2012, or specifically on the date December 21, except for the solstice, which happens every year.
None of the "predicted" happenings for 2012 hold up under close scrutiny. "Planet X" and "Nibiru" simply don't exist. The Mayan calendar ends a cycle, but there were no predictions of the end of the world. The Sun doesn't line up with the galactic centre; it's 6 degrees off. No asteroids or comets are actually predicted to hit Earth.
All of this stuff was put together by crackpots in order to promote their books and TV shows, and shamelessly promoted by the History Channel. Don't take any of it seriously!
The best way is to _inform yourself_ about 2012 by reading factual sites about it, rather than sites that are trying to sell you something. Here are a few:
http://www.msnbc.msn.com/id/33622934/ns/technology_and_science-science?pg=1#Tech_Doomsday
http://www.armageddononline.org/failed.html
http://www.2012hoax.org/
http://www.skyandtelescope.com/news/69774827.html
http://blogs.jpl.nasa.gov/?p=59
http://astrobiology.nasa.gov/ask-an-astrobiologist/intro/nibiru-and-doomsday-2012-questions-and-answers