Don't be... it is a bunch of nonsense...
A lot of people say that aliens are going to invade in 2012 and that a planet that isn't a planet its a brown dwarf star called Niburu will pass through our solar system at the same time and that's where the aliens come from.
This is a bunch of bull... the original source for this is the X-Files TV show.
BTW, if a brown dwarf star did pass through our solar system we would know about it way before now... and the pictures that people have proving it is on the way are false... you can't see brown dwarf stars... you can only see the absence of background stars.
Oh, and about the Mayan calendar... that's all it is... A Calendar. A highly accurate one, but it is still only a calendar... you don't see people freaking out and saying it is the end of the world when you don't see January of the next year after December of the current year.
As for what happens in July of 2012... Our solar system will pass through the 0 degree plane of our galaxy... we will go from being in the Northern hemisphere of our galaxy to being in the Southern hemisphere of our galaxy...
It is far more likely that the sun will release a massive solar flare that turns the earth into a barren cinder then we get invaded by aliens... if aliens were going to invade they would have done it by now.