Welcome home!
You will want to do an “examination of conscience” which is a series of questions or statements designed to help you identify your sins and areas where you need to focus on development. Usually these are based on the 10 commandments.
You can do a search on these forums or the Internet to find one. We recently had a good discussion about this with folks sharing links and complete examines.
You can certainly make a few notes for yourself to take to your first confession back. Please be sure to let the priest know how long it has been since your last confession (years) and he will guide you through.
Confess your mortal sins first, “in kind and number”. Kind being the type of sin, so all stealing, all sexual sins, etc. Number doesn’t have to be exact, but doe have to be as good an approximation as you can make it. So “I falsified my travel vouchers by $50 each time. I made about 8 trips a year for 10 years.” Or “I lived with someone I wasn’t married to for 5 years and we had sex regularly during that time. I also had sex with 10 other people over the past 5 years about once a week.”
You do the best you can with number and if the priest needs you to be more specific, he will ask. You don’t need to go into tons of detail about each sin or the circumstances around it. The priest doesn’t necessarily need to know who, what exactly you said, etc. It is absolution, not therapy. But if he does need clarification, he will ask. He may also cut you off if you start getting long-winded.
Your venial sins (not grave matter) will also be forgiven. It is a good practice to identify them and confess them regularly, but at this first confession after many years, please focus on your mortal sins and get them said, absolved, and off your conscience!