If you mean posting it on Fanpop, you should post it as an article. Chose the option 'Fan Fiction' when you are posting it. Obviously you should post it in the most appropriate spot, depending on the subject of your tagahanga fiction.
You have to have an account on fanfiction. YOu go to 'document manager' and upload your document. Then go to 'new story' and create a new story. I think you have to wait 24 hours before you can post one though.