The genre basics

What is it?: A contemporary YA novel is a story about a teenage character that takes place in our current time and deals with the concerns of modern day life.
Word count: The number is very flexible, but I would aim for at least 60,000 words. Stand-alone books tend to have a higher word count, like between 75 and 80,000 words. Series might come in closer to 60,000 words.

Notable authors in the genre: John Green, Sarah Dessen, Ally Carter, Ann Brashares, Meg Cabot
Recommended reads: The Fault in Our Stars by John Green, This Lullaby by Sarah Dessen (others of hers are good as well, but that remains my favorite of hers), the Heist Society or Gallagher Girls series by Ally Carter, The Sisterhood of the Travelling Pants series by Ann Brashares, The Princess Diaries series by Meg Cabot (both Sisterhood and Princess Diaries are tons better in book form than their silver screen adaptations), Anna and the French Kiss by Stephanie Perkins, There You’ll Find Me by Jenny B. Jones
Your questions answered:
Are contemporaries usually more focused on the characters than the plot?

Many of them are, yes. This is completely my opinion (and you should know that character building is my favorite and that contemporary YA is my genre of choice, so biases abound) but I think contemporary YA novels have a greater need for a strong voice and great main character because it lacks the bells and whistles of other genres. No flying monkeys, no teleporting. 
But there are high concept contemporary YA novels that have plenty of plot going on and don’t necessarily delve into characters as deeply as a story like The Fault In Our Stars or There You’ll Find Me. From the list above, I would say Heist Society and the Gallagher Girls series are high concept contemporary YAs, along with  The Princess Diaries series and the Travelling Pants books.

 In a fantasy or sci-fi story, you would probably say the antagonist is a bad person. But in a contemporary, the antagonist isn’t necessarily a bad person, they’re just not “for” the main character. Is that accurate?:

Yes, I think that’s true. In some contemporary YA series – the Gossip Girl books come to mind – the antagonist fluctuates depending on who our point of view character is as well as which book you’re reading.

Is the antagonist usually a person, or are they sometimes a force? Which usually works better to write, in your opinion?

In general, I think a person works best. Now, the antagonist in The Fault In Our Stars is cancer and time (I’m assuming. No character is coming to mind.) and that book has done all right for itself, so what do I know?

Also, do contemporaries often have more than one antagonist? Because in the Ellie books, sometimes Lucy could be doing something that kept Ellie from achieving her goals, but sometimes it was Palmer or Chase…

The writer who asked this question is referring to The Revised Life of Ellie Sweet and I thought it was a really perceptive question. In contemporary fiction, the antagonist does often seem to be a bit more mercurial than other genres. In a lot of my books, it’s the main character’s best friend who takes on the role of antagonist.
But I think it’s true for many great stories that there is more than one antagonist, even in other genres. Let’s look at Harry Potter because it’s easy. Voldemort is, of course, the big villain. But what about Snape? What about Draco? Dudley? They all take turns making Harry’s life hard.
So yes, it’s good to have a clear antagonist, but it’s also good to have other characters working against your main character at times.
Does the antagonist always have to be the one causing the inciting incident (not just in contemporaries but in fiction in general)? Or could the inciting incident affect the antagonist and cause him/her to become an antagonist?

No, the antagonist can cause the inciting incident, but I’ve never heard an argument that they should. That’s a personal choice about the story that you get to make on your own!