To unlock the City Planner trophy in Cities: Skylines, you will first need to play the game normally until you reach the Worthy Village milestone at a population of 240 - 950 depending on which map you are playing.
Once you reach the Worthy Village milestone, you will unlock the ability to create districts. Once available, select the district painting tool (found to the right of the zoning tool) and paint three different district areas on the map.
Once you have finished painting the third district, the trophy will unlock.