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First of all, let's define ionization energy. Ionization energy is the energy needed to remove 1 electron from each atom in 1 mole of gaseous atoms. The higher this energy is, the harder it is to remove the electron. The first main factor that determines ionization energy is nuclear charge (this is the relative charge in the nucleus of the atom and increases as the number of protons increases). As nuclear charge increases, Ionization energy increases. This is because the electrons are held more strongly to the nucleus by electrostatic forces of attraction between the positive nucleus and the negative electrons. Another factor effecting ionization energy is shielding, this is where electrons in orbitals closer to the nucleus 'shield' the valence electrons so they feel a smaller attraction from the nucleus, making them easier to remove. Across a period there is no significant change so this factor is not important. The third factor is the distance of the valence electron from the nucleus, as the distance increases, the ionization energy decreases, as the electrons feel the electrostatic forces less. Again, across a period there is no significant change in distance and so this factor isn't important. Although the last two factors don't effect ionization energy significantly across the period, they are still important to mention in your answer!

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