you would see your pastor and explain your situation to him and he will get you started on the process. he will also advise you if the marriage is invalid due to lack of form (example: married a non-Catholic without a dispensaton, or married civilly). And this would be a less involved process than a full-blown annulment.
there are canon laws that govern marriage, but an exhaustive list of possible reasons why a marriage would be invalid is beyond the scope of this forum. I suggest browsing the several past threads on this topic in the liturgy and sacraments forum, for great links on the topic, and for discussion of possible reasons for invalidity.
Canon law and natural law both govern marriage, and grounds may lie in either area.