The planet Nibiru was proposed by Zecharia Sitchin, based on his own translations of ancient Sumerian texts and his interpretation of Sumerian myths.
Actual evidence for the existence of "Nibiru" has yet to be observed.
As for 2012, there's also no evidence of anything happening on the date. Loony new age material will talk about galactic alignments and cycles, but it's all pseudo-science and has no basis on reality.
For starters, the solar system is always aligned with the center of the galaxy and also there's no reason to believe "alignments" have any kind of effect.
The solar system wobbles up and down while it moves around the galaxy. Some will talk about alignment with the galactic equator being the root of 2012, however the solar system crosses the equator at cycles of about 30 million years, while the supposed cycle for the Mayan calendar is just of 26 thousand years.
If the Mayan calendar is indeed of 26000 years, then it surely is based on the
precession of the equinoxes, a natural process with gradual effects (not noticeable with our limited lifespan).
#EDIT#
Found out now about
this site that details the flaws of Sitchin's work.