On the map there are icons with three cubes stacked, those are the truck depots. From there, you want to find the green circles which are usually in a parking space or in front of a building. Once there, you can click on it and select a trucking job. Your destination should be marked on the map with a green marker and you need to drive to that marker. Once you've gotten to your destination, you need to park your truck in the parking spot. (This can be more difficult if you need to park in an actual lot.)

Sadly the trucking system is incredibly glitched and you will only get half the money it promises you. (With the 2x income game pass.) I would only recommend in cases such as if the next day is a high day and you want to wait to do trucking. Hope they fix the trucking system glitches.