First of all, you’ll have to know what your sins are since your last Confession.
We get to know those sins in what we call the Examination of Conscience. No paper and pen involved. Much less, a calculator. But all the ‘answers’ should be the wrongs you did or the good acts you didn’t do.
So go to some place where you can focus and pray sincerely and simply. In front of the Blessed Sacrament is most preferable. Maybe in front of a crucifix.
Then you may use this examination guide if you’re an adult, or this if you’re a teen. Otherwise, you can just mentally follow the 10 Commandments and see how you might have disobeyed each (for example, regarding the First Commandment — which is on loving God above all things — perhaps you watched a movie which you knew was outrightly contrary to the Faith, and so on).
You may want to write down your sins — especially if you’re still quite new to Confession — but be careful not to discard your list such that others can find and read it afterwards. So it’s still best to find your method of memorizing your list of sins.
Finally, be truly sorry for your sins and promise God not to sin again, with the help of his grace. This is the manifestation of your contrition, and this is very important. Without this act of contrition and repentance, your Confession would be invalid and may end up being another sin — the sin of sacrilege, because the Sacrament has been treated merely as a plaything or a routine exercise.