Are you talking about a religious annulment such as the Catholic Church can grant, thereby freeing the couple to remarry? Alternatively, are you talking about a legal process, the outcome of which is an order stating that there was no legal marriage in the first place. (The couple would be free to marry anyone they legally could according to the laws of their country as though the annulled marriage had not taken place.)
Either way, you need advice from the authorities, civil, religious or both as the rules differ depending on where you live and what faith you follow. The rules also change from time to time.
Where I live, the sorts of issues that would lead a court to annul a marriage are that the marriage was entered into by fraud e.g. one of the parties did not realise that they were being married, and previously that the marriage was not consummated.
By the way, ‘you’, cannot annul a marriage. You need to make an application to the relevant authority for this to happen, and there is no guarantee that the court will make such an order.