Do you know that Mayans were good in mathematics and astronomy? why do you believe their prediction's wrong

Yes, they were good at math and astronomy. They predicted, exactly, that the 13th Baktun (a baktun is a period of 20 Katun, or 4000 Tun, or the same as 4000 years) will end on December 21, 2012. Of course, they didn't call it December 21.

They did not predict the end of the world. They predicted the start of the 14th Baktun. This is just the same as the prediction that December 31, 2000 would end the 20th Century and the next day, January 1, 2001, would be the start of the 21st Century.

Again, no prediction about the end of the world was made by the Maya.

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.