Your antagonist is extremely important to your story. The antagonist must be enough of a threat to bring your protagonist to his/her knees, to cause them to grow beyond what they could have ever done otherwise.
Antagonists can be of different sorts. There’s the well-known villain antagonist, like Darth Vader in the original Star Wars. A villain will want a goal that is completely at odds with that of the protagonist. Luke could either blow the Death Star out of the sky, or Darth could shoot him down first and save the day for the Empire.
Antagonists can also be a rival. If you’re writing a romance, the antagonist might be a man trying to steal the lovely heroine away from the hero. Of if your story’s a treasure hunt, the antagonist might be after the same priceless artifact, like the heart-shaped emerald in Romancing the Stone or the ark of the covenant in Indiana Jones. So in this case, the protagonist and antagonist both want the same thing, but they can’t both have it.
Your antagonist may actually be a group of people or an idea, faceless at first, although you’ll need characters to personify it in time. Dystopias do this well. Katniss starts out opposed to the Capitol in The Hunger Games. In time, President Snow comes to embody what Katniss is fighting, but the trilogy’s ending makes it clear that it’s more the idea of what the Capitol espouses that’s the real foe. (Look Ma, no spoilers.) In Orwell’s pivotal 1984, it was always clear that what the protagonist was fighting was totalitarianism itself.
Your antagonist may be nature, most commonly a natural disaster. Fires, floods, volcanoes, mountains–as long as it has the possibility of being lethal, it’s a worthy foe for your main character. However, notice that rain enough to flood the streets and people’s basements isn’t big enough to be an antagonist. You might be able to make it work as a plot complication, but it lacks the power to bring out the best in your characters for the main story.
Your antagonist may be within your protagonist him/herself. Addiction stories follow this path. Other opposing characters will have to be involved because watching someone navel-gaze isn’t normally going to carry even a short story, let alone over 50,000 words. But in the end, your finale will be the character defeating the destructive tendency within.
Regardless of the type of antagonist, you must walk a fine line. They must be threatening enough to keep the reader in doubt as to whether your plucky protag can defeat them, but not so out of your main character’s league that you must make your protagonist a Mary Sue in order for them to emerge victorious. A kid bringing down the school bully is believable (Karate Kid), but if they foil an underground criminal organization, you might have something like Scooby Doo. It worked for a Saturday morning cartoon, but it’s unlikely to carry a novel.
Put a great deal of thought into your antagonist. Conflict is the engine that will carry your story forward, so make certain (s)he is a worthy adversary.