First, are you on a Revolt or Conquest world? Just describe what image there is for the research Conquest in your academy.
If you are on a Revolt world, in order to take a ghost town, just send your Colony Ship with Light Ships, and land units (+ the Transports needed for the units). If you win the battle, then the city is yours. You do not need to have Conquest researched for this conquering system.
If you are on a Conquest world, you will need to have Conquest researched, then send your Colony Ship with Light ships and offensive land units (+ the transports for the units). However, if you win the battle, the city is not yours yet and you will need to wait for 24h (speed 1 world)/12h (speed 2 worlds)/8h (speed 3 worlds) during which you will have to hold a siege. In order to hold it, you will have to defend it (with Biremes and Defensive land units whoch should be sent as support so that they arrive few seconds before the CS landing). At the end of the 24/12/8 hours, and if you hold the siege successfully (without having it destroyed) then the city is yours.
If you still have questions, please feel free to ask! ;-)
