When you create a new InDesign document, just enter inches in the margin fields either as 1in or 1" (for 1 inch). When you leave the field it'll show the equivalent picas or points.
If you want to force InDesign to always use inches, select Preferences from the Edit menu, then select General. You'll see the dialog used to set Preferences. Go to the Units & Increments settings. You can change the units used to Inches and that will be the default in the future.