A Chelsea man pleaded not guilty Monday to charges that he fatally stabbed a man in Everett early Saturday morning, Middlesex District Attorney Gerard T. Leone Jr. said. Derren Molina, 21, was charged with first-degree murder in Malden District Court, said MaryBeth Long, a spokeswoman for Leone. She said that at approximately 1:19 a.m. Saturday, police responded to 194 Chelsea St. in Everett and discovered Louis Barbosa, 29, of Everett, suffering from multiple stab wounds. Barbosa was taken to Massachusetts General Hospital, where he was pronounced dead, Long said.