How to cut off the margins of the file with guides on printing

The page area is what prints, you also have the option to include a printable bleed area. Margin and ruler guides do not print or affect the print area. The document’s ruler units are set in Preferences>Units & Increments, or by control-clicking the rulers:

The easiest way to print part of a page would be to make a new document to the desired dimensions, place the larger InDesign page, and export or print the cropped page;

https://helpx.adobe.com/indesign/using/placing-graphics.html#:~:text=The%20Place%20command%20lets%20you,can%20add%20to%20InDesign%20documents.