, you need to unlock the quest: Trading post founder
which can be unlocked after unlocking several quests in Economic Development
(You can ignore this one if you play the game on the browser because you have more quests available)
Then, you need to research Trade Administration
and Trade logistics
The higher the distance you have to a special ressource on the map (e.g Papyrus), the higher are the ranks you need in Trade Administration and logistics.
YOU CANT attack an alliance or player who already has a trading post to get the achievement, so you need to find an empty
Simply click on the special resource icon you found on the map and click Trading post foundation and after a few minutes, your quest is done.
If there are any questions left, leave a comment and ill try to answer asap.
* Credits to Itzz Sh0wt1m3
for mentioning that you need Centralization 5
to be able to research Trade Administration and Logistics
** Request: Here is a visual example how a trading post looks like. The special ressource shown in the picture is Papyrus (there are several more like wheat etc.)