Right-click an empty cell, select "Format Cells," select the "Subscript" option and click "OK" to apply the style to the entire cell; everything you type in this cell will have the subscript property.

Google 101: how to format text in Google Docs. If you want to use strikethrough, superscript, or subscript, you have to find them first.

Office 2003 users must choose Font from the Format menu. It’s not hard, but some users will appreciate the built-in keyboard shortcuts for both: The keyboard shortcut for subscript is [Ctrl]+= ...