Jury selection has begun in the trial of a Boston man charged with the drug-related killing of a Maine woman. Prosecutors said opening statements in the first-degree murder trial of Amos ‘‘Ace’’ Don could start as soon as Thursday. Don is charged with shooting Erica Field, 29, of Lewiston, Maine, in the head in Dorchester in August 2009. Authorities have said that Don, 27, was a cocaine and heroin dealer who enlisted Field’s boyfriend as a dealer for the Lewiston area. Prosecutors said Don sat in the back seat of a car and shot Field and her boyfriend in the head as retribution for the theft of some drugs. The boyfriend survived.