This achievement can be got by just playing the game and gaining XP, but the easiest way to do it is, Go to Multiplayer and press Y to create a private match
Pick a car like the #1 Audi Team Joest R18 e-tron Quattro ( a fast car with good handling)
Then go to change options and then Game Options, then pick these -
Game Type - Circuit Race
Environment - Indianapolis
Track - Indianapolis Oval
Max Players - 16
Max Drivatars - 15 (That is what I prefer but you can do any amount as long as its more than 10)
Laps - 20 ( You can do more but 20 laps takes about 15 - 20 minutes to complete depending on the car)
Every lap you do is an extra 1,000 xp so 50 laps is 51,000 and 20 is 21,0000
The rest of the settings are whatever you want to put.
It takes a while to get it this way, but you will get other achievements along the way.

This video shows you how to level up and get money.

If you need help just PM me.