If you want to see the first time Luffy uses to knock out fodders:
After his two-year training with Rayleigh, Luffy uses Haki intentionally for the first time to take down the Fake Luffy and his crew without breaking a sweat. Massive Display (Episode 554): On Fish-Man Island, Luffy uses Haki to instantly knock out of Hordy Jones' 100,000-man army. Advanced Conqueror's Haki (Episode 1028): which episode luffy use supreme haki
However, a more notable instance is during the Marineford Arc (Episode 389) when Luffy confronts Akainu. If you want to see the first time
In this episode, Luffy's Supreme Haki clashes with Katakuri's, creating an intense and visually stunning scene. This marked a significant moment in the series, as Luffy's growth and mastery of his Haki abilities became apparent. Luffy's Supreme Haki clashes with Katakuri's