To unlock 'Los Angeles', you must sink 20 enemy ships with the USS Laredo.
The USS Laredo is unlocked during the third mission, and is available thereafter.
Some people seem to be confused about what counts for this, but simply put, all kills count.
* Kills made by the Loredo while under a ship control wild card.
* Kills made by the Loredo following your attack orders.
* Kills made by the Loredo in self defence (You didn't issue the order).
Basically, any kill counts.
The achievement is cumulative, so if you die, any kills you made before that death carry over after you reload.