Nocturne is highly dependent on his Level 6 ultimate; prioritize hitting Level 6 quickly and only gank or join fights when your ultimate is available.
Use your ultimate to target squishy carries, build a CS and objective lead, and aim to close out games fast as Nocturne falls off significantly in the mid-to-late game.
In team fights, focus on assassinating the enemy ADC. While farming for Level 6, be mindful of your lane's state to prevent them from falling too far behind.