This mission is given to you by Devin Weston ("D" on map) and can be started by any character. He wants you to steal two cars from people he knows, who just so happen to race them. Get in the marked vehicle and head to the gas station marked on your map. When you get there, Franklin will challenge the two guys to a race.
You just need to keep up with the two others, so don't go too crazy with trying to pass them up. You should be sticking to highway roads, so traffic shouldn't be too troublesome, although there will be other cars on the road. Remember, you're not trying to win, but just to lure the two guys into a trap.
Eventually, control will move over to Michael, who is dressed as a cop and will have command of a motorcycle. Your goal now is to get close enough to the two guys and pull them over. Click the left stick in to turn on your sirens if needed and you'll get them to pull over, where you'll steal the vehicles. You have your choice on who you want to drive to the garage to drop off the vehicles, where the mission will end.