Oh I was told something quite different from Tom.  I was told to confess everything I could remember.  I prepared for it by going through the 10 commandments and listing the ways in which I had broken each one.  Though of course, in doing that, you understand each commandment in a very broad, inclusive way, so for example, under "Thou shalt not kill", you're not off the hook if you've just never killed someone.  You must confess things like anger and gossip under that.  Then I also used the 8 virtues and vices, and looked for ways that I had failed to live up to the virtues/committed the vices.  It took a couple of days to get it all ready.  Good luck, and glory to God for your being joined to His body!
Oh, though it is true that it is not a sacramental confession, and so officially, you could leave things out and it would techincally be fine.  But as Elisha said, you lose a lot of spiritual benefits if you do that.  However, don't get overly legalistic, and just remember that all your sins will be forgiven in your baptism, whether you could remember to confess them or not. ÂÂ
And another warning: DON'T read the headings from the 10 commandments. It can lead to *misunderstandings*, like I said "murder", as a heading since it is one of the 10 commandments, and the priest freaked out and thought I had killed someone, lol.