The first ionization energy is the energy required to remove one mole of the most loosely held electrons from one mole of gaseous atoms to produce 1 mole of gaseous ions each with a charge of 1+.
The general trend is for ionization energies to increase across a period.
In the whole of period 2, the outer electrons are in 2-level orbitals 2s or 2p. These are all the same sort of distances from the nucleous, and are screened by the same 1s2 electrons.
The major difference is the increasing number of protons in the nucleus as you go from lithium to neon. That causes greater attraction between the nucleus and the electrons and so increases the ionization energies.