To itemize is to make a list. If you work at a store that sells pet fish, you might need to itemize your stock of fish — separately listing the number of goldfish, cuttlefish, and jellyfish.
When you place items, or individual things, on a list, you itemize them. Though itemize is essentially a synonym for list, it is usually used for more formal situations and those related to budget. So itemize your tax deductions or your spending report, or even itemize the reactions of your chemistry experiment. But to itemize your complaints about your little brother is probably going overboard.