Complex sentence: After his sixth form years, Firth moved to London and joined the National Youth Theatre, where he made many contacts in the acting world, from which he got a job in the wardrobe department at the National Theatre.
Split into simpler sentences: After his sixth form years, Firth moved to London and joined the National Youth Theatre. There he made many contacts in the acting world, from which he got a job in the wardrobe department at the National Theatre.