While reaching Driver Levels will come over the course of playing the career out and getting the other trophies there is an easy way to get to the maximum level of 50.
Create a quick race with the following settings:Mode: Drift
Car Class: Same Class
Track: MiyatomiAssuming you're using an automatic transmission, if using manual shift into reverse and use instead
With a Mazda RX8 (Costs $27,000) start the event and from the start line do a U turn and place your car so its against the outside wall, then Hold
continuously so that the front of the car is drifting away from the wall, let the race play out and repeat for the 2 'heats' of the drift event and you will have more than the required 500,000 for Level 50.
Here's a video created by BasSsFreAK1040