If there is a well known restaurant that is always packed for lunch, then definitely make a reservation. These can be made the same day from 11am to 12 noon or the day before. (it may be difficult to get someone on the phone in the height of lunch service)
If you have a lunch reservation, don't be more than 5-10 minutes late as this could really upset a busy restaurant.
If you don't have a reservation and the restaurant is busy, you may have to wait. Don't be surprised if you have to wait outside or just near the front door. (no waiting rooms here)
With or without a reservation, you might be requested to wait until the majority of your party arrives.