How can I write the formula for sodium sulfide?

The formula for sodium sulfide is ##Na_2S##
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Since this is an ionic compound, you must balance the charges so that overall charge of the compound is neutral.
Sodium, an alkali metal, has a tendency to lose one electron.
As a result sodium normally carries a positive one charge.
Sulfur, a nonmetal, has a tendency to gain 2 electrons.
This results in an ion with a negative 2 charge. Nonmetal ions end in “ide”.
To obtain a neutral charge, you need two sodium ions, which provides you with a plus 2 charge balance the negative 2 charge of sulfur.
Therefore this is the chemical equation.