If you receive your first baptism, you do not need to confess, as you are cleansed at that point. If you were baptized earlier in life, you will need to confess before receiving first communion. This confession should go back as far as your baptism. This should be addressed at RCIA and time should be spent on this, both to help you understand what is going on and to help you prepare for that confession.
You need to confess all the mortal sin you remember (and should really work hard to remember as much as possible). Mortal sin are those grave sins that you commited with full knowledge they were grave sins. IOW, things you now know are sinful that you did not know were sinful at the time might not need to be confessed. That said, it doesn't hurt to err on the side of caution and confess those things that might be "on the line."
It isn't as important to be able to detail every instance of a particular sin as much as to be able to say what it was, how serious it was, how often it took place, and how recently. IOW, you don't have to go back and remember the name of every boy or girl you looked at with lustful thoughts.
Hopefully, it won't be something to be nervous about, but something to anticipate eagerly. Look to it as an opportunity for freedom.
This has everything to do with the OP, as well. Guilt plays an important role. It shows us where we have strayed and (assuming we listen and are obedient) leads us to repentance and confession. Once we have repented and confessed, the guilt has done it's job and there is no reason for it to linger! We need not continue to feel guilt for stuff in our past that has been properly dealt with.
Penance may include "making things right," but not always. Some things can be put back together with ease. In many cases, though, you can't unring the bell, so to speak. Vascectomies are in the middle. In some cases, the right thing to do might be to have it reversed. In other cases, it shouldn't. If in doubt, talk to your priest and/or spiritual director and listen for the Holy Spirit speaking to you.
Yes, a vascectomy is a sin. No, that does not mean you are cut off from God with no chance for redemption.