Once I'd built a few roads, I zoned Foozle's residential, commercial, and industrial areas. Unlike previous SimCitys, zones border roads instead of filling areas, and density is defined by the size of the zoned road.
High density streets zoned for residential construction will result in apartment buildings, for example, and small streets will become suburbs. Eventually, the commercial properties on my main avenue will become much bigger, as long as there's room for them to grow. This gets tricky: if you pack in low-density roads early on, you may have to demolish them later to make room for bigger buildings.
I didn't plan for Port Foozle's future very well, but there's always the next attempt, and the attempt after that. Starting a city is like rolling and re-rolling an RPG character to get just the right mix: it's almost as addictive as actually playing the game through.