How can I write the formula for magnesium sulfide?

Hint: For writing the molecular formula of magnesium sulfide you have to know about the valence or oxidation state of magnesium (${\text{Mg}}$) atom as well as of sulphur (\[{\text{S}}\]) atom or sulphide ion.

Complete answer:
Some points regarding magnesium and sulphur are as follow:
-As we know that atomic number of magnesium is $12$ and electronic configuration of magnesium is ${\text{1}}{{\text{s}}^{\text{2}}}{\text{,2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{,3}}{{\text{s}}^{\text{2}}}$.
-And from the electronic configuration of magnesium it is clear that ${\text{Mg}}$ will lose two electrons from its outermost shell to get stable electronic configuration and shows positive oxidation state i.e. $ + 2$.
-As we know that atomic number of sulphur is $16$ and electronic configuration of sulphur is ${\text{1}}{{\text{s}}^{\text{2}}}{\text{,2}}{{\text{s}}^{\text{2}}}{\text{2}}{{\text{p}}^{\text{6}}}{\text{,3}}{{\text{s}}^{\text{2}}}{\text{3}}{{\text{p}}^{\text{4}}}$.
-And from the electronic configuration of sulphur it is clear that \[{\text{S}}\] will gain two electrons from outside in the outermost shell to get stable electronic configuration and shows negative oxidation state i.e. $ - 2$.
-Sulphur in $ - 2$ oxidation state is known by the name sulphide.
-As the name magnesium sulfide suggested that sulphur is present in sulphide form or in the $ - 2$ oxidation state.
-It is clear that magnesium is present as a cation which is also showing $ + 2$ oxidation state to get joined with sulphide which acts as an anion.
-Chemical formula of magnesium chloride is written as ${\text{M}}{{\text{g}}^{{\text{ + 2}}}}{{\text{S}}^{{\text{ - 2}}}}$.

Hence the formula of magnesium chloride is ${\text{MgS}}$.

Note:
In this question some of you may write the wrong formula of magnesium sulphide if you are not able to write the electronic configuration of magnesium atom as well as sulphur atom for calculating the valencies.