:up: 'The first thing' means what it says. Without the article, 'first thing' means 'the first thing in the day' [=very early]. So 'Our flight is at 7.00, so we'll have to get up first thing'. And it doesn't have to be early in absolute terms (about dawn); it can also refer to the beginning of, say, a working day: 'All right, you don't have to do it now, but you must do it first thing tomorrow'.
b