The best answer to your question is that you should first make an appointment with your parish priest. Each person’s situation is different and each annulment case is unique; your priest will work with you through the process and guide you through the steps that are necessary for you.
As you are considering an annulment process, another preliminary step is to gather the documentation necessary to present an annulment case. This includes the civil marriage license, the civil divorce decree and any baptismal certificates, if either or both of the former spouses are baptized. If you are seeking an annulment of a prior marriage because you wish to re-marry, your intended future spouse will also need to provide these documents, if applicable. Bring these documents with you to your meeting with the priest; he will explain to you what else is needed and help you to submit the required material to the diocesan marriage tribunal so that the annulment process can begin.