Chinese Astrology is based on a twelve-year cycle as opposed to the twelve-month cycle of Western astrology. It has its origins in the legend that Buddha called together all the animals in creation, but only twelve of them turned up, and those that did were each given a year, in the order of their arrival. Just as Western astrology presupposes that your character is influenced primarily by the astrological sign that the sun was in at the moment of your birth, so Chinese astrology decrees that your character is influenced by the animal sign governing the year in which you were born. .
The date of the Chinese New Year varies from year to year, and it generally occurs during the second half of January or the first half of February. If you were born during this time of the year then you will need to consult a Chinese astrologer or date book to find the Chinese year of your birth, and if you were born in the first half of January, your sign is that of the previous year.