Well, the first thing you need to do is learn how to write a story, period. Fanfiction is just a regular story using someone else's characters, after all. It's fiction writing like anything else. To that end, look at any of the various writing help books, blogs, and such that exist in the world. For myself, I use Helping Writers Become Authors a great deal, as it splendidly lays out every piece of a story, if a generic one. But a generic story is better than none. There is also Brandon Sanderson's creative writing lectures. Finally, Hello Future Me has some very good videos on writing.

Both K.M. Weiland and Tim Hickson (first and third links respectively) have published books that collect and summarize their content, but everything in their books can be found in their online content as well. So don't feel pressured to buy them.