Trucker
To begin working as a trucker, you must first buy a phone from a hardware store, then use the job center app to change your current profession to "Trucker". Then, make your way to the Los Santos port to pick up your truck. There is a marker on the ground inside the warehouse there. When you walk up to it, a small box should appear on your screen Make sure you stand close to the marker on the ground when you're trying to spawn the truck. You will be charged a $250 deposit when you spawn the truck, which will be returned to you once you return the truck to the spawn point.
Once you have selected a truck, you will automatically be given a waypoint which shows the drop-off location. Once you get to this location, you should get an option to "pick up products" when you stand near the back of your truck. Take these products and drop them off at the marked location (usually near a door). Repeat until you can no longer take products from the truck, and you get a notification telling you to move to the next drop-off point. You will continue to receive drop-off points until you have visited every shop in Los Santos, at which point you can return to the port and collect your pay slip at the desk with a laptop on it. Pay slips may take some time to deposit into your account, sometimes requiring the player to relog.
Truck Logistics
At this location, you are able to buy additional trucks, recruit drivers, assign those drivers to different trucks, take out loans, and repair your trucks. Unfortunately, most of the options in this menu is currently in Spanish.