Holden Caulfield is the protagonist throughout the book. He is the main character and the novel is given from his (first person) perspective. Since the novel is told by Holden directly to the reader, the antagonists are presented from his perspective. And the antagonists are quite simply the "phonies" of the world. These are the people that Holden is against. In this way, Holden clearly establishes who he thinks are the antagonists of the world and in his life.
Grouped in with phonies are those who sell out. He praises his brother, D. B., for his earlier writing, but condemns him for going to Hollywood to be "a prostitute." Holden also has a strong dislike for movies, being quite literally phony as well. Holden felt that Pencey's headmaster (Mr. Thurmer) was also phony and praised his daughter (Selma) because she didn't brag about being the headmaster's daughter:
What I liked about her, she didn't give you a lot of horse manure about what a great guy her father was. She probably knew what a phony slob he was.
In Chapter 2, Holden shows his respect for Mr. Spencer but doesn't want to stay and be reminded of how he'd not applied himself in school. To his credit, Holden isn't resentful about flunking out of school. In discussing his predicament, Holden reflects on one of his previous headmasters, Mr. Haas as "the phoniest bastard I ever met in my life." So, as much as he despises phonies, Holden also despises authority figures. This is common for a teenager but Holden is even more intolerant than the typical teenager.
The opening line of Chapter 3 suggests some hypocrisy in how Holden looks at the world and himself. Although he sets up his antagonists as phonies, he freely admits that he is phony himself (although, he does not use the word "phony"):
I'm the most terrific liar you ever saw in your life. It's awful.
For someone who despises phonies to the extreme, it is quite hypocritical to be a "terrific liar," a direct indication of being phony. A lot of Holden's criticisms of others may be valid, but he doesn't shine the same light of criticism on himself. So, to Holden, the antagonists are phonies. But in the larger context of the book, and Holden's life, he is his own antagonist. This is part of his maturation; he looks at the world critically, even intelligently sometimes, but he thinks he is somehow above it all. (This superior position is alluded to at the beginning of the book when he is alone on the hill, looking down at everyone else at the football game.)
Holden has nothing but good things to say about his brother Allie, who died young and therefore never had the chance to become a young man or an adult, thereby dying before he could become a phony teenager or adult. This is one of many instances where we see Holden equating youth with innocence and genuine character, as compared to equating adults with phoniness. Despite Holden's hypocritical tendencies, he is basically aware of his own faults. He mentions how he tends to act younger than he is and he freely admits he is a compulsive liar. Given that he equates adults with phonies, and that he equates youth with honesty, there is part of Holden that wants to stay young and avoid the phony, adult world. And yet, this book is about Holden attempting adult things. This is the dichotomy. This is why Holden is in a state of transition; confronting the phonies of the world while reconciling his own phoniness.