Complex sentence: Barton began his career at age 19 as a scoring assistant and additional conductor for noted Hollywood film composer Harry Gregson-Williams at Gregson-Williams' studio, Wavecrest Music.
Split into simpler sentences: Barton began his career at age 19 as a scoring assistant. He was an additional conductor for noted Hollywood film composer Harry Gregson-Williams at Gregson-Williams' studio, Wavecrest Music.