One of your first stops should be Barfleur Tourism Office. Here, you'll be able to orient yourself and get lots of useful information before you start exploring.
This Barfleur tourist information center is a good place to start your day of sightseeing. There's plenty to discover close by, including Tatihou Island Maritime Museum, Tatihou Fort, and Gatteville Lighthouse. Take your time at Barfleur Tourism Office before making tracks to these other fine sights.