Multiple container ships can be found at Los Santos Harbor and are available to rob. They can be found on the map with the following icon: 
In order to rob the ships, you must get to the checkpoint located near the bridge. Once there, type /robbery to begin robbing.
After this, you will start burning the door lock and a timer will appear in the information box on the right side of your screen. Note that police will be notified about the robbery, so keep an eye on the map for incoming cops.
After you're done robbing, you must escape from the ship. Grab a vehicle and get to the hideout. Sometimes your hideout will be on an island, so make sure you have the right vehicle.
Once you've reached the hideout, the robbery will be complete.