XSL:FO determines the formatting of an XML document, and is way outside the current workflow for regular InDesign usage. It may (just) be possible to create a custom script that writes out certain elements of an InDesign document in XSL:FO syntax -- the data from paragraph and character styles, for example. But an XSL:FO stylesheet that mimicks the exact layout of an InDesign document is technically not feasible, as InDesign has many options that are not supported in XSL:FO renderers.
Without a corresponding XML document, a single XSL:FO file does nothing at all. Exporting properly tagged XML out of InDesign is a standard built-in option; see the online Help for an overview of that: InDesign Help | Working with XML