I come up with a story concept first and I work out how it will end before I begin writing.
For example: PLOT: A cop's wife is found guilty of murder and going to be executed for the crime. The cop is willing to do anything to stop this, including framing an innocent man of the crime his wife is accused of. ENDING: Cop is arrested for framing man and loses his job. Wife is executed for the original crime. TWIST: The man the cop was framing really was guilty of the murder.
How do you prep for a story?