Complex sentence: His career as a journalist began in his second year at high school, when he established "The Progress" newspaper; later, he worked for the "Haddonfield Press" and for the "Camden Courier-Post".
Split into simpler sentences: His career as a journalist began in his second year of high school, when he established "The Progress" newspaper. He later worked for the "Haddonfield Press" and for the "Camden Courier-Post".