John Steinbeck's classic novel of Depression-era ranch-hands struggling to eke out a living has two main protagonists: George and Lennie. Of Mice and Men tells the story of these two individuals as they seek steady employment during a period of history when unemployment affected tens of millions of Americans. There is a complicating variable, however, that provides the novel's most compelling drama. Lennie is mentally handicapped. Despite being an adult, he has the mind of a young child. This is not a problem in and of itself; what makes Lennie's condition dramatic, within the context of Steinbeck's narrative, is Lennie's enormous physical strength--strength he cannot control and, when combined with his childlike inquisitiveness, invariably leads to trouble.
George is Lennie's more physically diminutive friend. George is smart and has been taking care of Lennie for a long time, including helping the latter avoid angry mobs when Lennie's carelessness and curiosity have been misinterpreted (which is quite often). Throughout Of Mice and Men, George laments the burden of taking care of Lennie, but he sticks with the mentally disabled man because they are all each has. George curses his misfortune to be shackled to this big, lumbering child whose lack of social skills and overpowering strength repeatedly lead to catastrophes. Whenever Lennie asks George to discuss their dream of a ranch of their own, however, George complies. The smaller, wiser man resents being a caretaker, but he remains committed to his and Lennie's friendship.
The main antagonist in Of Mice and Men, other than the abstract notion of the Great Depression imposing hardships on the two protagonists, is Curley, the spoiled son of the owner of the ranch. Curley's bellicose nature and physically attractive young wife provide the basis for the tragedy that will come as the novel moves to its conclusion. Steinbeck makes it very clear early on in his novel that Curley represents a threatening presence in the lives of the ranch-hands, including the two newcomers, George and Lennie. Note, in the passage that follows, how the author introduces his antagonist to George and Lennie, newly arrived at the ranch that will be their home:
"His eyes passed over the new men and he stopped. He glanced coldly at George and then at Lennie. His arms gradually bent at the elbows and his hands closed into fists. He stiffened and went into a slight crouch. His glance was at once calculating and pugnacious."
Curley is surely the principle antagonist in Of Mice and Men. His presence at the ranch is a constant source of tension and his jealousy regarding his attractive wife, combined with what the reader learns about Lennie's earlier encounter with a girl in a soft dress in a previous town, sets the stage for the fateful developments to come.
Of Mice and Men has quite a few characters that could fit the bill for protagonist and antagonist. As a whole, the ranch hands are your "good guys," while the owners and associated family are the "bad guys."
Specifically, your best bet for protagonists are Lennie and George. The novel starts with them and ends with them (although it doesn't exactly end on a high note). The story focuses on their struggles and their dream to own their own farm. While neither of them is perfect, the reader can't help but want to see them succeed.
Specific antagonists would have to be Curley and Curley's wife. Curley is mean. He is abusive to those around him, he picks fights frequently, and abuses the power he has because he is the boss's son. Curley's wife is an antagonist because she is a temptress who attempts to lure the men away from their goals. While she isn't totally "evil" (the reader almost feels she is a victim in some ways), she doesn't help the protagonists out in any way.