To get the Cleric stronghold, choose Cleric as the class for your main character. Go to the Temple district in Athkatla and you'll quickly find someone recruiting people to their cult. After the dialogue, a priest will ask you to come talk to them in their temple. (Your alignment will decide whether it's a priest of Helm, Lathander, or Talos.) Enter the priest's respective temple, and they'll give you the quest "Investigate and Destroy the Cult of the Eyeless"(also known as "Find and Destroy the Cult of the Unseeing"). Go to the sewers and find the cult location. Complete the quest, then return to your temple to speak with the priest, and they'll offer you a place in their temple. Accept, and the achievement will unlock.
Note: This can be done immediately after the prologue and speaking with Gaelan Bayle.
Note 2: The cleric basically has a choice of three different strongholds based on their alignment. The stronghold side quests are the same for each, but they have completely different solutions to them depending on your temple.