An antagonist is a character who acts as an opponent to the protagonist, the central figure in a novel. The antagonist may be a person, or it may be a larger force, like the environment. There might also be a number of different antagonists in a story, at different points in the protagonist's development.
The protagonist in Slam! is Greg "Slam" Harris. Slam's coach, who recognizes that Slam has amazing talent in basketball but thinks that he is a "prima donna", works as an antagonist against him. The coach mockingly chides Slam for not being a team player, disrespecting him frequently before his teammates and keeping him on the bench.
Other antagonists in the story include Mr. Parrish, Slam's English teacher, who blatantly makes fun of Slam's African-American heritage in front of the class when Slam forgets a homework assignment, and Ice, who acts as a foil to Slam's confused attempts to rise above his life in the ghetto.
In a larger sense, however, the antagonists that work against the main character's development in this book are the influences of his neighborhood and the anger and pride within himself which keep him from reaching his full potential. Although he attends a relatively "good" school, Slam must fight to overcome a background defined by racism and poverty. Slam has a loving family, but even within that framework are forces that hold him back, as exemplified by his father's drinking and his resentment at the idea that his son might receive tutoring from someone he perceives as being better than him. Slam's greatest obstacles to fulfillment, however, lie within himself. He knows he possesses outstanding skills on the basketball court, but struggles to reconcile the fact that the world does not owe him a living because of his talent; Slam must learn that there is much more to getting on in life than playing ball.