Decide who the main characters of your story are. The protagonist may be the ghost, or a human character. The antagonist may be the ghost or a human character as well. The antagonist doesn't need to be evil or scary, but simply a person who is causing problems for the protagonist. Create a small cast of characters, including the ghost, before you begin to plot.
Write down a sentence or two describing the plot of your ghost story. The basic structure of any story can be broken into three parts; the catalyst (what spurs the main character into action at the beginning of the story), the main conflict of the story, and the resolution or ending. This can be very basic to begin with; for example, "a boy notices strange occurrences in his house and investigates, discovers a ghost with a problem, and helps him solve that problem."
Develop each of those three parts into more detail; perhaps a paragraph or two for each part. Keep your story interesting by asking questions and creating more conflict. For example, the first part in the previous example was a boy who notices strange occurrences in his house and investigates. What are the strange occurrences? Maybe he realizes the food in the pantry is rearranged every morning. Why is this a problem? Maybe the boy's father is a chef who gets upset if anyone messes up his kitchen. Now the boy has a reason to prove that it is not him causing the disturbance. Introduce another problem; the boy's father absolutely does not believe in ghosts. Now even if the boy catches the ghost playing in the kitchen, he'll still face the conflict of getting his father to believe him. Continue asking questions and introducing problems for all three parts of the sentence from Step 2.
The ghost in your story might be friendly, but if he is not the antagonist there should be another one. In the example above, that would be the father. Write a plot summary that answers the following questions: Why is the ghost in contact with the main character? What is the antagonist's problem? What is the main conflict the main character needs to resolve, and how does he resolve it?
Write a first draft of your story based on the plot summary. Choose either first person ("I"), or third person ("He"). Grammar is important, but allow yourself to write in your natural voice, inserting humor when appropriate. Read your draft out loud, then revise any awkward sentences and fix any plot problems.