Navigation

If by "Navigation app" you mean the Maps app, then just open the app, enter an address or location to search for, then when it finds the location tap on the "Directions" button, select your method of travel and the tap on the "Go" button.

On an iPhone, it should use GPS automatically to calculate route and navigate once you are on your way.

See here for more.: Use Maps on your iPhone, iPad, or iPod touch - Apple Support

If navigation app is Apple Maps, start here:--> View maps on iPhone - Apple Support

then clic on "next" at bottom right corner

Regards

Giulio