Why would you even skip the the first episode of Demon Slayer.
Its a 26 episode series. Just run it through from start to end. It does start off a little slow, but its well worth slogging through the beginning bits.
The movie is a direct sequel to season 1, but you could get away by watching the movie standalone i guess.
My suggestion: Binge season 1 if you got time to prep for the movie (it should all be on netflix now). Otherwise just watch the movie blind and come back to the first season at your own leisure.