I think some new converts or others believe you have to tell every mortal sin you can think of since early childhood, but for new members I think two or three issues is enough to tackle on the first time--that will be plenty of time to talk to the priest and develop a comfort level.
And after confession ALL past sins are forgiven ,even ones not mentioned. I usually say something to the effect when nearing the end of my list: And for everything that I cannot now remember, but want to be forgiven with a sincere heart.
It really is painless and it draws you closer to Christ.