If you're actually in the process of writing fan fiction or are planning to, then please for the love of god ensure you have the following down.
Basic punctuation: Start of sentence needs to be capitalized as do names and places. Use commas to break up sentences and to give the reader a break or when witting long descriptions. Additionally you should use ! ? appropriately along with ; to elaborate on a sentence that would otherwise not make sense on its own, and lastly every sentence needs a full stop or else it won't make sense. Yes that seems easy, but if that's true then why is the majority of fan fiction riddled with such basic errors?
Basic grammar. Know the difference between to and too, your and you're, affect vs effect, their and there, it's and its, plural and possessive, were and we're etc.
Can spell: If you can't use spell check or get a beta reader, or better yet don't write at all.
Sentence structure - linked into punctuation and is related to making proper and decent paragraphs.
Can write dialogue and yes it needs to be punctuated too, it also needs to be separated into new paragraphs when a new speaker begins speaking.
And lastly, no more isn't better, which means you shouldn't cram every single cliche in your given genre into your story, because you know a well written and developed plot about a simple idea can be great, so no you don't need to insert every single Fanon idea into your story to make it better, because it most likely won't.