You can see your speed in the top left hand corner. This achievement can be achieved in episode 2 or higher. Most of the cars featured in the daily event in episode are able to reach 300kmh+. Simply pick up nitro boosts, make stunts and knock down cars until you have a full pink nitro bar. Then use nitro by pressing the space bar and get the achievement.
Don't worry if you don't get the achievement in episode 2, you''l definitely get it in the higher episodes, where cars reach 300 kmh without using nitro.