Divorce, in and of itself, is not an impediment to becoming a Catholic. You would not need an annulment to become Catholic.
You would only need an annulment if you are already re-married or are seeking to get married again.
If your marriage to a Catholic was in the Catholic Church or received a dispensation to be married elsewhere then it is a presumed valid marriage and you would need a formal annulment.
If you married a Catholic outside the Catholic Church without a dispensation then the marriage would be considered automatically invalid and you would simply need to present evidence of the situation and you would obtain a simple decree of nullity due to lack of form.
