Characters in fiction are motivated by all sorts of things. How they act and respond to the world around them is determined by these very motivations. A sure way to sink any story is by creating a character who is static, or who isn't motivated by anything to do anything. And in order to write a good story, the character must DO something to create the action in your plot. Before writing the plot to your story, determine how and what will motivate your character. Here are some steps to get you started.

Define your character.
What type of person is he? Is he timid? Is he headstrong? Is he aggressive? Or is he a passive aggressive personality? Is he independent or is he a lonely person? By creating a personality trait for your character, you'll have a better sense of why your character is motivated to pursue his goals. For instance, a lonely character in a love story might be motivated by entirely different reasons for the choices he makes than an aggressive, headstrong and independent character.

Create a back story for your character.
How did the character become the way he is? Write down his family history, how he was raised and any events in his past which shaped his life and outlook on it. You don't necessarily have to include all of this information in your story, but it will help you to maintain a level of consistency in how your character will behave and what motivates him to behave that way. For instance, your aggressive character might have grown up in an aggressive environment, one in which he had to become tough in order to survive.

What is your character's outlook on life?
Is he angry at the world, or is he an optimistic, happy-go-lucky guy? A character's outlook can reflect what motivates his actions in your story. For instance, if your character is a businessman who believes that the only way to success is through hard work, then that character's motivations will be reflected from this outlook. It can also affect how he reacts and responds to the world around him. If his son is a lazy slacker who wants to leech off his old man's money, then the businessman will have a very specific reaction to his son, and will be moved to respond in a certain way. His motivations to raise his son to be like him will come directly from his own outlook and experiences in life.

Give your character something he desires.
Whether it's wanting to find true love or to get rich, all characters must yearn for something. Without that yearning, then you have no story. Using your character's personality traits, his background, and outlook on life, determine what it is your character will want in your story. For instance, the lonely man will of course want to find true love and companionship. The businessman will want his lazy son to validate his personal belief system. While these wants can seem, on the surface, somewhat shallow, your character, with his richly defined background, will add layers to your character's motivations. The lonely man may have grown up in a dysfunctional family and is estranged from them. His need to find companionship then will become all the more acute because he yearns for it so desperately.