We've all been there, with a plot or charecter in mind, but unable to actually start writing a story. What you need is some story starting writing prompts!
Do you have a story idea, but you are having a hard time actually start writing it? This is, for some, the hardest part of writing fiction-- beginning. What you need is a good writing prompt to kick-start the creativity and to get that novel or short story written!
Think about your idea, whether is based on a person, a series of events, or a place. What do you hope to accomplish with this story. Will it be scary, pulse pounding, poignant, or comical? If it is character based, what is your character going to learn? Or, in other words, how will the character change throughout the story?
Now, you know what the story is about, but how do you start it?
Well, the key here is just to start it. Chances are that you will come back later and change the beginning a little, or even a lot. But in order to revise, you actual have to write!
With a character driven story, start writing about their life up to that point. Most of what you write may not even make it into the story, but doing a rough-sketch of them on paper will allow you to create a more believable character, and (gasp!) you will be writing!
-What was the scariest moment in your character's life?
-What events have had profound effects upon them?
-What are their phobias and fears?
-What makes them smile?
-What sort of personality do they have?
-How would she react in a crisis?
The key here is in the tiny details. You don't need a ton of detail, but concise is better than obtuse. For example, which of the following pairs of lines would tell a reader more about a person? (The answer should be obvious!)
She is a happy person.
-Or-
She is the type of person that could stand in front of her tornado-destroyed trailer and laugh because at least she doesn't have to fix the leaky roof anymore.
He is reckless.
-Or-
He once drove down Main Street, at high noon, naked and hanging out of the driver's side window, while steering the truck with his right foot and using a hockey stick to press pressure on the gas pedal.
She loves to shop.
-Or-
He can always tell that she's been shopping by the way she greets him at the door. A smile and a hello means that no money was wasted that day. A quick kiss means that she's shopped, but not enough to make him cringe. And if she meets him at the door wearing nothing but a dazed expression and a lusty grin, well then, he's broke but about to get lucky.
Even if it has nothing to do with your story, put your character to talking about himself, or place him in unfamiliar territory and see how he reacts. You never know, you might end up with a gem, and at the very least, you will succeed in starting to write your story.