1. How will this cause doomsday?
It won't. Since "Planet X" doesn't exist, it can't have any effect.
2. What will it do to destroy earth or at least humans?
Absolutely nothing.
3. Where is this planet right now
I have been asking this question for a year now, and not one person has been able to give me its coordinates. If it were following the orbit ascribed to it, it would be a bright naked eye object in our sky by now.
4. Is this really a planet, or just a big obstacle in space?
Neither -- it doesn't exist.
5. What is you're input on "Planet X"?
It is a myth put together by crackpots to sell books and TV shows.
6. Any other facts about this planet i should know?
It doesn't exist -- what more do you need to know than this?
Scientists don't expect anything out of the ordinary to happen in the year 2012, or specifically on the date December 21, other than 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!