The only restaurants you need reservations in advance are Emiril's and Tchop Chop. For the other restaurants, you can make a reservation at the stand located at the entrance of City walk. I have a picture of it at the bottom of my page. Go there the day you plan to visit, say you want to make a reservation, and they'll give you a card. For all restaurants, the most crowded time is 8pm. If you are staying onsite, your room key will move you up the list. Otherwise, it's best to get there between 5-6pm, or go for lunch at 11:30am.
If you want a party type atmosphere and don't mind hanging out at the bar or walking around City walk for half an hour, go at 8pm. At Margaritaville, it's a party like you wouldn't believe and the wait is 1 hour to get in. Stilt walkers go thru the crowd, people singing, volcano going off.
Everyone basically does the same thing, they visit the parks, go back to the hotel and rest, then decide to go eat at 6pm. By the time they get out of their hotel room, and make it over to Citywalk, it's 8pm and the place is packed. Go early than this, unless you want the party atmosphere.