In case you don't know, and some people don't and that's okay, Protagonist is a fancy word for Main Character. Harry Potter is a protagonist. Katniss Everdeen is a protagonist. Also referred to as the hero/heroine of your story.
Most books have only one protagonist, but if you're writing a book from more than one point of view, then each point of view character is a protagonist. Pauline Hawkins does this in THE GIRL ON THE TRAIN alternating between three characters. I also do this in the PERCEPTION series, alternating between Zoe and Noah, and in A Nursery Rhyme Suspense series alternating between Marlow and Sage.
The antagonist is the bad guy. The obvious antagonist in HARRY POTTER is Voldemort.
Often in any given story the protagonist faces more than one antagonist. Harry Potter also has a lesser antagonist in Professor Snapes.
In HUNGER GAMES, Katniss's main antagonist is President Snow. Her lesser antagonists are her competitors in the games.
But your antagonist or antagonists are not only other humans or human-like forces. An antagonist is anyone or anything that gets in the way of the protagonist's quest or goal.
The antagonist can actually be a situation like a war or bad weather if its the protagonist’s hurdle.
And very often, I'd say almost always, there is something called the Inner Antagonist. The inner antagonist is a belief system that gets in the way of the protagonist’s goal or quest. In HARRY POTTER one of Harry's inner antagonists is his belief that he can't be the one chosen to conquer his outer antagonist, Voldemort. He is constantly questioning his call and abilities.
In THE GIRL ON A TRAIN, Rachel, the main protagonist is an alcoholic. This flaw is her inner antagonist, leading her to believe that she deserved every bad thing that had happened to her, and also to blame others for every bad thing that had happened to her.
In PERCEPTION, the antagonist is Zoe's Vanderveen's grandfather, a powerful and intelligent man. Her inner antagonist is her initial belief system—that GAPs, Genetically Altered People—are superior to non-GAPs.
When you are plotting out your stories, make sure you understand who your antagonists are, both inner and outer, and how they are going to work towards thwarting your protagonist’s quest or goal. And in the climax, your protagonist must conquer her outer and inner antagonists.