Before you start a fan fiction, it's best to have at least some idea of where you are going. Some people have all of their fan fics plotted out beginning to end, others just go with the flow until they feel it is completed. Do whatever works for you, but when you write fan fiction- or indeed, any story- write it out beginning to end. Do NOT go back and revise it until you have a first draft complete. Then you can go back and make it perfect. It makes a better work in the end.
A few helpful links to look at:
Some users on this wiki would be willing to look your story over before you post it publicly. They make suggestions, catch mistakes, and edit grammar and spelling. These users are called Betas. It is a smart idea to get a Beta. You know what they say- four eyes are better than two!
When you start a story, you may want to put it in the appropriate place in the Story Order.