If you believe your marriage was invalid, you must petition the tribunal in the diocese where the marriage took place or the diocese where your former spouse resides. The tribunal where you reside could handle the case under certain conditions. Your pastor can help you contact the appropriate tribunal. A tribunal is the official Church court of a diocese which handles marriage cases.
The most common grounds for an annulment are lack of due discretion, defective consent, and psychic incapacity. (You can follow the article link below for more on those three.) The tribunal can assign you someone to help you prepare, and you can also go to any canon lawyer for help with your petition. You pastor can help you locate a canon lawyer.
Although some people object to the process of collecting evidence and conducting a trial, it really is necessary to help people resolve their doubts about the validity of their marriages. If the first tribunal declares the marriage null, an appeals court in a second diocese must uphold it.
If you are poor, the process costs nothing. If you have limited means, you pay as much of the fee as you can. If you have means you pay the full tribunal fee. Fees differ from one diocese to another; check with a local parish or the tribunal office to see what the fee is in the diocese where you need to present your case. These fees come nowhere near meeting the Church’s costs for processing annulments, which include paying the salaries of the Tribunal staff.