As noted, all you need to do is go to Confession. While you could go at a regularly scheduled Confession time, I highly recommend making an appointment because your Confession is bound to be long if you have 20 years or more of sins to confess. I estimate your Confession will take 30 minutes to an hour based on my own experience making a General Confession. Making a list is encouraged but not required. It is normally encouraged to confess your most serious sins first, but in your case, it is probably better to list your sins in chronological or reverse chronological order, or go through the Ten Commandments in order and list your transgressions of each Commandment. If you need help, the priest will help you through your Confession.
After your Confession, I encourage you to attempt to make a plenary indulgence: which consists of receiving Communion, praying for the intentions of the Pope, removing all attachment from sin, and completing an indulgenced work (Confession is also required, but your General Confession satisfies this requirement). In addition to indulgences available on specific days, there are several plenary indulgences available year-round: half an hour of Eucharistic Adoration, half an hour reading/listening to the Scriptures, praying the Rosary in a group either in church or in a family setting, a pilgrimage to a Patriarchal Basilica in Rome, or a retreat lasting at least three days.