If someone new to film noir wants to learn about it through a single film, Scarlet Street is the one of the ones I would possibly hand her. It's pretty much perfect.
The great, often unsung Edward G. Robinson is Christopher Cross, a quiet, creative man with a mind-numbing bank job and a nagging shrew of a wife. He falls for Kitty, a young, pretty (not to mention loose, foul-mouthed and not particularly bright) woman who mistakes him for a famous artist. Kitty's abusive, domineering boyfriend Johnny sees Cross as a sugar daddy, and pushes Kitty to push Cross for money, which Cross, unaware of Johnny, starts stealing from the bank so that he can keep Kitty happy. It doesn't take much to make him happy - when Kitty and Johnny start passing off Cross's paintings as her own, to their own profit, Cross is so pleased that his work is appreciated he lets her continue to do it. And when Cross's wife's deceased husband suddenly turns up alive, he sees an out of his unhappy marriage and goes to convince Kitty to marry him. Kitty, however, laughs in his face, enraging Cross to the point that he snaps and murders her with an ice pick. Johnny is executed for the murder, but Cross loses his job due to his theft, and tries to kill himself. Unsuccessful, he ends up penniless and alone wandering the streets with Kitty's voice echoing in his head.
Scarlet Street illustrates most clearly the lesson all film noir whispers from the shadows: what you desire can destroy you. Every character in this film wants something desperately: Kitty wants the love of her boyfriend (who is really not much more than a thinly-disguised pimp) so that she'll do anything to keep him, but her game with Cross ends in her death; Johnny wants the money that Kitty can get him, but his involvement results in his murder conviction; Cross's wife wants her deceased husband back, but when he does come back, he's revealed to be crooked and corrupt. And Cross - he wants a lot. He wants freedom from his wife, he wants to be appreciated as a painter, and he wants the love of a beautiful young woman. He gets it all. He even escapes conviction for a murder he committed. But once he's lost everything he gained, he is a great deal worse off than he ever was before, and the loss drives him literally insane.
The film benefits greatly from having a master at its helm - director Fritz Lang, to put it succinctly, knows what he's doing. Every shot, every movement, every line, is deliberate, and carefully builds the characters and story. The performances are brilliant, too.
Scarlet Street is in the pubic domain, and is all over the internet - unfortunately, it's also very poor quality. I've embedded a clip below, but it wouldn't be a bad idea to seek out the 2005 remastered DVD to get the full effect.