As with everything in this game, it will take time. You will start with residential buildings, but as you obtain Great Buildings or Event Buildings that have population awards, you will be able to start eliminating your residential buildings, until you have either zero, or just specific ones that you may want to keep (i.e. Piazza Home - if you want to have the Piazza set constructed in your city). And as Klods references above, you will also be able to start eliminating goods and supply building (if you wish to play that way) because you could have certain Great and Event Buildings that will provide those as well.