GTA 5 Online offers you many vehicles and the Cerberus truck is one of them. If you want to get hold of this truck, then there are some restrictions which are as follows:
You must have an auto workshop
There must be 2 or more players in the free session
You cannot travel to a vehicle spawn location with a Helicopter, Airplane, Jet, Opressor MK2/MK1, Deluxo, Special Vehicle, or Amphibious Vehicle
If you have met these requirements, then you just need to go to one of the spawn points of this exotic truck. To know where you can go, here is the map.