After finishing the Renting a Garage job, talk to Sheriff Black to start this one. Talk to Rusty at Rusty's Autoparts. You'll have to win some races, so talk to Jackie in front of Wellspring Speedway. Then talk to Slim in the booth to select a race. After the race, use your Racing Certs to buy some weapons for your buggy. Then head back to Sheriff Black and this will start the Resupply Dan Hagar job.