If possible, start with the Memorial de Caen - a visit here will give you an overview of World War II and the part the Normandy D-Day Landing Beaches played. Housed in a modern purpose-built structure on the outskirts of Caen, the huge exhibition takes you from 1918 and the damaged peace through World War II and on to 1989 and the fall of the Berlin Wall. the museum is very well laid out so you can take it in sections, spending as much time as you want in each part.
The memorial is full of objects, models and films made both during the war and afterwards and you are carried easily into the history of the war and the stories of the soldiers. Particularly impressive sections include Total War, the 1941 events such as the attack on Pearl Harbor that took the war from Europe to the rest of the world. You should allow at least half a day; a whole day is best. But don’t try to do more than the Memorial in a day; it’s comprehensive, packed with stories and events of the war. Not only is it best to absorb all this before taking in more of the Normandy sites; a visit here is sobering and you may be left feeling drained.