Choose the main character's conflict or problem. Just like choosing your title, character or setting, the plot you set off on can make or destroy a great story. This will be easiest if you have already chosen your main character for your story.
Brainstorm a list of plotlines you would find scintillating to read about. Try your best to avoid the standard stereotypes of 'boy meets girl, boy loses girl, boy wins her back' or 'character finds herself by moving away to New York.' Anything you have read before, your audience has too.
Consult potential readers. What would they enjoy reading about? When not possible, compare your plotline to your setting. Do they go well together? Would it be realistic to have this problem in this setting? Choose whether or not your plotline is believable.
Write out steps as to how this problem developed for the character. Did they walk right into a bad situation? Did it happen over time? Write out steps the character will use to escape the problem. If you can successfully do this, you have a storyline. If you cannot prewrite these steps, all you have is an idea with potential that should be saved for later. Choose another plot.
Providing you can sketch out the steps in and out of your story's trap, you have yourself a plotline. Congratulations and start writing.