Characters, in my experience, tend to be the key elements in any book.
They are the driving force, the element that readers empathize, sympathize, boo and cheer on.
So how do you get started? Think about your story, think about the character.
What are your characters strength, weaknesses and growth opportunities? List them all out.
Chart them or write little mini stories, what ever works for you.
Another important subject to address is the characters history.
Was he/she/it an only child? One of many? orphan? foster child? Did your character grow up rich or poor or in between? Good parents or bad parents? Failed relationships? Was your character abused? Does the character have loads of experience or if he naive? The very smallest details can monumentally change your character.
Perhaps your character saw someone die, or lost something precious or became sick from eating an apple.
Everything can add to your character, little details make the character, favorite colours, foods, ways of speaking, parents, siblings, test scores and hobbies.
Who? Who is your character? What? What profession? Ethnicity? When? In what time if your character? The present? Future? Where? Does your character live on Earth? In Belarus? LA? Has the character moved? Why? What motivates your character? Why is your character the way he/she/it is? How? Link it all.
How did it all happen? Etc In the end it all comes together and creates a unique personality, a person that your reader can see, touch, feel and relate to.
To either love or hate.
What you will likely notice as you write is that your character will become more defined, somethings that you have planned may fail and others may grow or sharpen.
A good character grows, develops and matures, so as it changes add or change the lists, charts or paragraphs that you create to realize your characters.
They learn from their mistakes (You learn along side them).
You don't want your character constantly falling into the same exact trap every time.
A great thing to do is find a picture, a photo or perhaps have an artist friend draw a picture of your character.
It will make him/her/it more real to you.
Also be sure to do something similar for your other characters, not just you heroes.
It makes them far more three dimensional.
I have also noticed in my writing and that of others, that each time you write a story (or novel) you are redefining a character, you create an archetype which you develop more and more as you continue your writing career.
Don't expect to be perfect from the get go, give yourself and your characters room to grow.