On an air map, Zerg have a great advantage because of Mutalisks. The most common thing people do with zerg is upgrade to spire as quickly as possible, get mutalisks, and upgrade their attack and armor. There are lots of minerals that are left over from doing this, and those minerals can be spent on anything including spore colonies, zerglings, and hydralisks. Overlord speed and transporting needs to be upgraded eventually, to make expanding possible. As zerg, you must keep your enemy from expanding, using your mutalisks all over the place. Normally it's a bad idea to attack an opponent's base directly, because they will have defenses for air. Eventually upgrading to guardians is necessary, and they should always be accompanied by many mutalisks and overlords if the opponent uses cloak. Queens can be used if an enemy is defending their town with templars. Parasiting shuttles and dropships makes them almost completely useless, so do that often.