There are 3 parked pizza boy locations but 2 are on the western island which is initially locked. The first available location is at the Well Stacked Pizza company just north of the Malibu Club. You will find the pizza boy in the alley in between buildings. Once you get on you will get 6 pizzas and 1 customer with the number of customers increasing by 1 each run. If you have more than 6 customers in later levels you will need to revisit the shop to get more pizzas and continue.
You must drive by the customer and throw them the pizza like you were doing a drive by. If you miss and the pizza is too far away the customer will not pick it up and you will not get credit. This can be fixed by getting on/off the scooter and pushing the box to the customer but will take more time. I recommend you plan your route and hit the furthest customers in one direction first. You only need to deliver a total of 10 pizzas for the trophy. However the game does not give you a count and the trophy will not unlock until you complete level 10 or fail/quit. If you complete level 10 you will get a permanent health increase to 150 instead of 100. You also need to complete level 10 for 100% completion.