Most cruises stick to the southern Norway fjords (see map below) because they are the most popular. Other cruises do include some northern Norway destinations in their itineraries (especially in the North Cape and Svalbard Islands). Some cruises even add Iceland.
Ports of call
Bergen is the gateway to the Fjord region. Its old town and harbor are interesting. However, except for their surroundings, the other ports have middling tourism appeal, so don't expect much. And you shouldn't. The allure of western Norway's coast is its stunning natural scenery, not its villages and towns.
Many major cruise lines operate Fjords of Norway cruises. The Norwegian Hurtigruten company runs the most.
The range is several days to weeks. Seven days is common.
What to wear
Dress in layers. For shore excursions, wear good-traction shoes, dress casually, and bring raingear (rain in common). Dinner code is smart casual.