I work in a company where I have to wear a necktie. I heard that wearing a tie is a sign of Christians? What should I do?

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions.

The necktie originated in Europe. It is said that it was a symbol for the obedience of the husband to his wife. Then it became what it is now. Wearing has become a habit for many Muslims and is no longer a tradition just for non-believers. If the believer wears it only to imitate the westerners, reference to it is included in the sound Hadeeth "Whoever imitates a people, he is one of them". So it is safer for the Muslim to avoid wearing it unless this could cause him some harm, such as being fired from his job. So, if not wearing a tie might cause you harm at work, you can wear it.
Allah knows best.