As a truck driver, you need to ensure that the cargoes will reach its destination. If possible, there should be no cargo left behind, but will depend on the truck you are driving so you just need to deliver the required cargoes to its destination. That way, both your boss and his client will be happy. Your boss will be proud of you and might give you a raise. But, if you aren't given a raise then that is not the end of the world as you still have your job. What is important is that you are able to do your job right and not some reward that your boss has installed for you.
Drive your truck to the loading area and wait till the cargoes are properly placed on your truck. Once, the cargoes are well-secured, you need to drive your truck and deliver the cargoes into its destination. Try to be quick as there is a time limit. If you manage to do deliver the right amount of cargoes then you will proceed to the next level, but if you manage to deliver more than enough cargoes then you would not only proceed to the next level, but also get more money. Save your money until you have enough to buy a better and bigger truck. You can play the same level over and over again until you are satisfied with the results.