First you need to do a thorough examination of your conscience. There are multiple websites that can give you a good guide to this, but basically you reflect on the Ten Commandments and the Seven Deadly Sins and try to determine which mortal sins you’ve committed and how many times you’ve committed them. I’ll occasionally write down my sins, sort of a Reconciliation cheat sheet, although I generally leave this in the car and don’t bring it in with me.
Next, you actually go to Confession. You should decide in advance if you’re going to do private or face-to-face. I was mortified with the idea of face-to-face Confession until I started doing it. Now I couldn’t imaging doing it anonymously, but to each his own.
Once you enter the confessional, you’ll both make the sign of the cross and then you can start by saying something along the lines of, “Forgive me, Father, for I have sinned. My last Confession was ___ days/weeks/months/years ago.” You can expand on this if you feel it’s appropriate.
The priest may then say a prayer or read a passage from scripture. Then again, he may not. The last time I went was very informal and actually skipped a lot of the above.
He may then ask you to go on with your Confession, or he may just stare at you expecting you to know what to do next. Either way, this is where you start confessing your sins. You’re obligated to confess your mortal sins, although you can also bring up venial sins, especially if it’s something you really struggle with. You’re supposed to be able to give the number of times you committed each sin, but that may be difficult especially since you haven’t been in a while. The times I’ve gone after having not been in a very long time, the priest was okay with a ballpark figure. Again, though, you’re mileage may vary.
Following this, the priest may offer some advice/encouragement or ask some questions, and then he’ll give you your penance. In my experience, penances now aren’t what they were when I was a kid, and it’s usually nothing earth-shattering. Yet again, your mileage may vary. Then he’ll likely ask you to say an Act of Contrition. I never can remember the Act of Contrition, so I bring a little prayer book with me. If I forget it in the car, I pray they have a copy on hand. Other times, though, I’ve had the priest do it with me, question and answer style, which is always a relief due to my mental block on this prayer.
The priest will then absolve you (followed by the sign of the cross), possibly read again from scripture, say a prayer and tell you to go in peace. You’ll then walk out, do your penance and hop in the car, remarking about how much easier that was than you thought it would be, and wondering why you put it off for so long.