Do Dereks achievement first and foremost as this achievement "Proper Scrapper" will come in handy later in the game once you are earning money to pay people back.
To get this achievement all you need to do is go to his scrapyard in a crap car, crush it using the controls walk down the steps and do the right (across from his shop) there is a car parked there, take it and go straight towards the train tracks and to the left, usually there are two cars there, take one, turn around and come back, the first car you stole should be there. Crush the one your in and repeat the before process. Do this until you have done 5 cars.