Character development is so important and is an aspect of writing beginners tend to forget about. Forgetting about this key detail can also lead to writer's block later on,  if you just rush into the story and forget to actually develop your characters. 

Think about people in real life. We are very complex beings, full of emotions, likes, dislikes, skills, flaws, etc. Make sure your characters are like this, and not just one-dimensional.  Really take the time to go through your characters and give them life. Make them exist. 

What I personally do with my characters is make a separate document with points about each one. List their friends, enemies, teachers, parents, attributes, personality traits--anything you can think of!

Here is a (shortened) personal example, if you don't know what I'm talking about:

Metro Riverton -- Character Notes:

-very smart, academically and socially 

-born a Class A "Purebred"

-Parents Athena & Nelson Riverton

     -Athena is socially smart

         -wants Metro to be perfect

        -wants to look good to public

       -doesn't care to show affection

  -Nelson is theoretical physicist 

      -socially awkward

     -relies on wife

A really good way to further advance your characters is by preforming an interview with them. 

Pretend you are a talk show host or a reporter or a magazine. Ask your character a variety of questions, as specific as you'd like to be. I'd recommend staying away from questions like "What is your name?", "Where were you born?", etc, because that's really not helping you develop your character. Make sure to ask questions you don't know the answer to. This will help to really give your character some depth, dimension. Actually believe that your character is real. If your character isn't real to you, they're not going to be real to your audience.

Here is another personal example if you don't know where to start:

How would you describe yourself?

I am a highly intellectual individual, academically and socially exceptional. I stand out among my peers because of my ability to read body language and detect the emotions of others. I am opinionated, although I do not always share my opinion. I am able to formulate my own judgments through personal research and experience.

What is your musical taste, if any? Any favorite artists in particular?

Although loud music is strictly prohibited in my living quarters (close proximity with other students could arouse a conflict), I do find myself leaning towards rich vocals and acoustics or piano. I highly enjoy the modern, artificially made sounds created by DJs, such as music by Alan Walker or any other Proximity artist.

What do you fear? How do you escape these fears?

Although it is slightly embarrassing to admit, I fear authority and being scolded for my wrong-doing. I fear discipline. I am not used to being accused for serious trials. I escape them simply by avoiding them. Running away. 

Remember, your questions don't have to have anything to do with your story. They can be completely random. The point is just to develop your character more, which also develops your plot.

Here are some other good questions to ask your character:

Describe your closest friends. What do you like about them? What do you dislike about them?

What conflicts do you face at the moment? How do you plan to resolve these conflicts?

If you could only eat one food for the rest of your life, what would it be?

Where do you go to be alone? How often do you go there?

What is one thing on your bucket list?

How would you get away with "the perfect crime"?

Describe yourself as an animal.