how did he start his musical career?
Upon joining a local singing group called the 3 Flashes.
Frank Sinatra
He got his first break in 1935 when his mother persuaded a local singing group, the 3 Flashes, to let him join. 
who said the greatest thing you'll ever learn It is the last line of a song written by Eden Ahbez. where is the above mentioned writer from? He was born in Brooklyn ,New York. what is his famous work? For composing the song "Nature Boy".. when was this song composed? UNANSWERABLE what are some versions of this song? The Dick Haymes recording, a version by Frank Sinatra, another version by Bobby Darin and many more. where was the second mentioned person born? 415 Monroe Street in Hoboken, New Jersey. where did he study? UNANSWERABLE who were his parents? Natalina "Dolly" Garaventa and Antonino Martino "Marty" Sinatra. what were their professions? His mother was a midwife and his father was a bantamweight boxer in the beginning and later worked in the Hoboken Fire Department.
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Frank Sinatra [SEP] Hoboken Four, Harry James, and Tommy Dorsey (1935–1939)
Sinatra began singing professionally as a teenager, but he learned music by ear and never learned to read music. He got his first break in 1935 when his mother persuaded a local singing group, the 3 Flashes, to let him join. Fred Tamburro, the group's baritone, stated that "Frank hung around us like we were gods or something", admitting that they only took him on board because he owned a car and could chauffeur the group around. Sinatra soon learned they were auditioning for the "Major Bowes Amateur Hour" show, and "begged" the group to let him in on the act.