Although this achievement may sound obvious, finding the missions which will get you there, isn't.
I drove around for an eternity without being given a mission until i figured it out.
After selecting your location, in the car list you will see some cars have a red flag next to them. Those are the cars which have current missions. If there are no red flags, hit B and swap to/from city/autobahn and you will see more missions.
Generally, you will be given a mission soon after loading the map. There are a handful of missions, though, which require you finding the red ring/arrow to start them.