Driver Level 50 Achievement
Driver Level 5020
Reach Driver Level 50 in Career mode.
As you play through Forza Motorsport 4 you will earn "Driver Level" and everything you do - online races, event select, and rivals mode, etc. - all contribute to this rank. All you have to pretty much do then is just play the game normally and this will come if you are going for the event completion or season achievements.