Let us revise ionisation enthalpy in the following points-

1. The first ionisation enthalpy of an atom is defined as the energy required to remove completely the outermost electron from a gaseous atom in its ground state

2. As electrons are negatively charged and protons in the nucleus are positively charged, there will be an attraction between them.

3. The greater the pull toward the nucleus, the harder it will be to pull an electron away from an atom (and ionisation energy will be higher).

4. The energy required to remove one mole of electrons (to infinity) from one mole of gaseous atoms to form one mole of gaseous positive ions. 

5. The value for helium is higher than that for hydrogen because there are now two protons in the nucleus. The nuclear charge is greater so the pull on the outer electrons is larger. More energy will be needed to pull an electron out of the atom. 

6. Lithium atoms have 3 protons so you would expect the pull on electrons to be greater. Lithium 519 kJ mol-1 However, the 1st Ionisation Energy of lithium is lower than that because the further away the electron is from the nucleus, the lower the attraction between the electron and the nucleus. In addition, filled inner shells have a shielding effect which decreases the attraction.