Hi Michelle. Welcome to cruisereviews, and congratulations on booking your first NCL cruise.
As the others have stated, you'll have a choice of various restaurants while onboard, and no set dining time. As Cortney said, you don't need to pay for dining unless you want to. On our most recent NCL cruise, which was 7 nights long, we dined in the (free) main dining room three times, the buffet once, the (free) 24-hour comfort food restaurant once, one night in the ($10 per person) Italian restaurant, and one night in the ($15 per person) Asian Fusion restaurant.
I do suggest that you make reservations. Depending upon your cabin type, this can be done 24 to 48 hours in advance. (Even more time if you're in a suite.) The Teppanyaki Room fills up quickly... that was the only restaurant on our cruise that was regularly filled to capacity. There will also be monitors throughout the public areas of the ship indicating how busy the various restaurants are, and how busy they expect to be the entire evening based on previously-made reservations.
This is not to say it's perfect, yet we enjoyed it... and I am a traditionalist when it comes to cruising!