’s first notable use of Supreme King Haki (Conqueror’s Haki) occurs in Episode 389
"Thunder Bagua!" Luffy roared.
Episode 478 : Luffy used conquerors haki and man wtf (1st watch notes) : r/OnePiece. which episode luffy use supreme haki
Conclusion
However, if you mean the first time Luffy uses basic Conqueror's Haki to knock out weak-willed people, that is Episode 413 (at the Sabaody Archipelago, after the Straw Hats are separated by Kuma). ’s first notable use of Supreme King Haki