How to get nullification decree from court

Assuming you are hindu,
The grounds for a marriage annulment may vary according to the different legal jurisdictions, but are generally limited to fraud, bigamy, blood relationship and mental incompetence including the following:
1) Either spouse was already married to someone else at the time of the marriage in question;
2) Either spouse was too young to be married, or too young without required court or parental consent. (In some cases, such a marriage is still valid if it continues well beyond the younger spouse's reaching marriageable age);
3) Either spouse was under the influence of drugs or alcohol at the time of the marriage;
4) Either spouse was mentally incompetent at the time of the marriage; 
5) If the consent to the marriage was based on fraud or force; 
6) Either spouse was physically incapable to be married (typically, chronically unable to have sexual intercourse) at the time of the marriage;
7) The marriage is prohibited by law due to the relationship between the parties. This is the "prohibited degree of consanguinity", or blood relationship between the parties. The most common legal relationship is 2nd cousins; the legality of such relationship between 1st cousins varies around the world.
8) Prisoners sentenced to a term of life imprisonment may not marry.
9) Concealment (e.g. one of the parties concealed a drug addiction, prior criminal record or having a sexually transmitted disease). 
from the breif facts you've provided us we recommend you to look for the grounds that fit into your situation and proceed with the annulment accordingly.