The shooting happened about 11:30 p.m. Saturday in the 9900 block of Anzac Avenue.
Police said 31-year-old Jesus Orosco confronted two gang members and told them to stop flashing gang signs at his 18-year-old stepdaughter's birthday party.
"During the party, 19-year-old Luis Padilla became involved in a dispute with another party member," said Cleon Joseph of the Los Angeles Police Department in a statement. "When Orosco tried to intervene and stop the confrontation, Padilla pulled out a gun and shot Orosco."
He was then shot once in the chest and died from his injuries.
Police found Padilla in the Jordan Downs Housing Development early this morning. He was arrested and booked for murder. The teen is being held in lieu of $1 million bail.
Detectives said they are continuing their search for more possible suspects.
If you have any information regarding this shooting, you're urged to call police at (877) LAPD-24-7 or (877) 527-3247.
City News Service contributed to this report.