The atomic number of Lithium is 3. This means there are 3 protons, positively charged particles, in the nucleus of a lithium atom. There are always the same number of electrons in an atom as the number of protons, if the atom has an overall charge of 0. Thus, there are three electrons in a lithium atom.
When an atom loses an electron, the atom becomes a positively charged ion. This is because now the number of the positively charged particles, protons, exceeds the number of electrons, so the net charge is positive. In the case of lithium, if one electron is lost, the resultant ion has two remaining electrons, and a net charge of +1:
The net charge of three protons and two electrons is +3 + (-2) = +1.
If two electrons were lost, the resultant ion would have the charge +3 + (-1) = +2.
Therefore, the lithium atom has to lose one electron to form an ion with a +1 charge.