As the potential jurors gathered, Judge Debra Nelson denied a defense motion to delay the trial, NBC News reported.
"We're not fully ready," defense lawyer Mark O'Mara had told the judge.
George Zimmerman, a Hispanic-American, was charged with second-degree murder in the shooting death of Trayvon Martin, 17.
"We ask that the community continue to stay peaceful as we place our faith in the justice system," Martin's father, Tracy Martin, told reporters before the proceedings began, later saying, "We ask that you continue to pray for Trayvon and our family."
Six jurors, plus alternates, will hear the case. Jury selection is expected to take about two weeks.
Zimmerman, 29, admits he shot Martin Feb. 26, 2012, but claims his actions were in self-defense because the teen attacked him. Zimmerman has pleaded not guilty.
As jury selection begins, some evidentiary matters remain unsettled, including whether jurors will hear from expert analysts about a 911 call taken on the night of the shooting, NBC News said.
O'Mara said he hasn't decided whether Zimmerman will testify.
"That is a call we always make midway through," O'Mara told The New York Times.
The jury pool is about 500, NBC News said.
About 100 to 200 potential jurors were expected in court Monday, with 100 more each day after that until the panel is seated, Clerk of Courts Maryanne Morse told the Orlando Sentinel.
Zimmerman, who was not arrested until six weeks after the shooting, has maintained he fired his 9mm semiautomatic gun in self-defense after Martin attacked him.
Prosecutors said Zimmerman provoked the confrontation by profiling Martin, who was walking back to his father's girlfriend's home, minding his own business.