Peace be with you…
Praise God that you have come to realize the great joy and benefit of the Sacrament of Reconciliation.
The biggest and most important advice is “don’t worry about it”. God does indeed know your sins already and also knows your desire to repent, recommit and to grow in holiness.
Your confessor is bound to have dealt with this situation before, so just go in and do the best you can.
As a general form, I was taught that you go in, kneel (or sit), make the sign of the cross and begin:
“Bless me Father for I have sinned. It has been (a really long time) since my last confession.”
Then you list the sins you can remember. The norm is that you should list type and number, but since you haven’t been in a really long time, I would simply suggest that you confess that you committed the various sins “numerous time”, or “habitually”.
During and after this Father might ask you a question(s) and will give you a penance to do. You will then say an act of contrition:
"Oh my God I am heartily sorry for having offended Thee,
and I detest all my sins because of Thy just punishment.
But most of all because they offended Thee my God,
who are are all good and deserving of all my love.
I firmly resolve, with the help of Thy grace, to sin no more
and to avoid the narrow occasion of sin.
Amen.
After this your confessor will absolve you and grace will enter, cleansing you in a wonderful way. You will leave the confessional wonderfully refreshed.
Peace
James