Ionization energy is the energy it takes to remove one electron from an atom. The higher the ionization energy, the harder it is for it to lose an electron.
Rule 1. Remember the rules about electronegativity? The trends for ionization energy are almost the same as electronegativity, because the more attractive you are to electrons, the harder it is to remove your electrons.
Rule 2. Ionization energy trends have a few exceptions, based on the structure of atomic orbitals. The s, p, and d orbitals like either being completely full or exactly half full. Therefore, when an orbital has one more electron more than being completely or half full, the atom finds it much easier to lose that "extra" electron. This causes a discrepancy in ionization energies as seen in Na or Co.