A teenager narrates the entire book so the usage of slang is quite understandable.
Many of the slang expressions in it are probably nowadays considered to be old-fashioned and not used anymore.
Examples:
“So I got the ax (from the school)” to get ax = to get expelled
“So I shot a bull for a while.” to shot a bull = either to lie and exaggerate or to chat amiably
“Boy, was he sore.” sore = angry, irritated
“What really knocks me out is…” to knock someone out = to delight or impress someone extremely
“So I went down to the can and chewed the rag with him”
can = toilet
to chew a rag = to converse
“I’ll be up the creek if I don’t get the goddam thing in by Monday.”
up the creek = in trouble
“Some of the ones he said were flits were even married, for God’s sake.”
flit =male homosexual
“I mean he didn’t hit the ceiling or anything.”
to hit the ceiling = to become violently angry

Story-line:
It is Saturday evening and there is a football game at Pencey Prep School. Everybody is there except Holden. He’s found out he’s been expelled from school again so he goes and says good-bye to his history teacher. He is supposed to stay at school until Tuesday when the Christmas holiday starts. But after he gets back to his room and finds out that Strandlater has left for date with Holden’s old friend Jane, he decides to leave the school tonight and spent couple of days in the city (N.Y.C.). He is fed up with the school life and people that surround him. He gets into fight with Strandlater and afterwards he packs his suitcase and leaves the school disappointed. He takes a taxi to the station and gets on the train to the city. Here he meets a woman, who turns out to be a mother of one student from Pencey, which Holden happens to know. He makes up some stories about the woman’s son just to impress her. Afterwards he regrets it. When he gets to New York he goes to a hotel to spend the night there. He realizes he doesn’t want to sleep yet so he decides to head for down town to the club. Here he meets some young ladies, who according to his opinion “are not quite as good looking”. However; he invites one of them to join him for a drink. But as soon as she starts talking he dislikes her and decides to leave the club. Although he is under age he goes to another club where he asks for some (alcohol) drink. He meets one of his brother’s ex-girlfriends, but doesn’t feel like talking to her since his brother lives in Hollywood now and Holden doesn’t want to explain her everything. He takes a taxi to go back to the hotel. The elevator man offers him a prostitute for 5 dollars. He accepts the offer and goes to his room to get ready for the “lady”. When she comes he is very nervous. He reveals he is still a virgin although all the time until that moment he has been talking about the girls as if he’d had some experiences with them. He doesn’t really want to have sex with the prostitute. He just needs someone to talk to. But the girl doesn’t understand him, so she leaves after taking the money. The next morning, when he wakes up, he is really depressed and thinks about committing a suicide. After a while of thinking he calls his ex-girlfriend and makes a date with her. He still has a lot of time to spend until the date, so he decides to spend it by just wandering in the streets. He takes his suitcase and puts it to “one of those boxes” on the station. He decides to have breakfast at the station.