Every writer has their own style. The best way to discover yours is to just keep writing, experimenting with different things, until you find what you're comfortable with.

A few pointers:

1- I (SP) write using "(Y/N)," which is an acronym for 'Your Name.' It is designed to allow the reader to place themselves in a story. However, some people may find this limits the development of their original characters (known colloquially as OCs). You will have to experiment and find whether you like using the inclusive acronym or naming your protagonists yourself.

(a) Other, similar acronyms include: (Y/L/N) = Your Last Name, (Y/H/C) - Your Hair Color, (Y/E/C) = Your Eye Color, etc.

2- Many younger or inexperienced writers will write stories that follow what is termed a 'Mary Sue' model. This is when a thinly-veiled version of the author, generally idealized, is placed in the universe/environment of the subject of the fan fiction. Often, their romantic attentions are vied for by other, known characters within the universe. This is also termed 'self-insert.' Mary Sue stories are very common and often have a very poor image among 'fanfic connoisseurs,' but there is nothing wrong with using personal experience to make your story more relatable. Finding a balance between boring, indulgent tropism and creating protagonists with depth is something that comes with time.

(a) i'd wager a huge majority of most folks' first fanfic stories are at least partly mary sue. -a

3- Become acquainted with a thesaurus (either online or a physical thesaurus). Overuse of a single word makes reading boring for the reader. Use a thesaurus to find synonyms for what you are describing. It will also help you expand your vocabulary. If you are just starting, you might want to cross-check your chosen synonym by its dictionary definition as well.

4- Character naming. All I'm going to say is that a lot of fanfic authors have really stupid names for their characters. I have no idea why. If all the names that come to you are profoundly stupid, I suggest using (Y/N).
*Note- You can always look up what names were common for the given historical time period you are writing in. Alternatively, you can use baby naming websites, and even look up names by their meaning.

5- Don't simply write 'skips to [next scene/area].' It's totally ridiculous. Take the time to write out the transition or imply it within the text.

6- Chapters should be composed of story. Chapters are not supposed to be lengthy author's notes. Do not write chapters begging for attention for your personal problems, especially if you are not receptive when concerned people offer you legitimate resources to help. Do not write chapters simply apologizing for not updating. Do not write chapters asking where your readers think you should go with the story.

(a) this is my personal fucking pet peeve! if you have mental health problems, there is nothing wrong with that. but threatening to hurt yourself in the middle of a fucking fanfic story is totally fucked!!! -a

7- Trigger warnings are a controversial issue. In my opinion, they are most warranted in the lengthier description of a story (not the short blurb, and certainly not your story's title), and perhaps in each applicable chapter's description. If it is for something truly terrible, you should ask yourself why you are writing about something like that in the first place.

8- Pictures of outfits. I personally hate this. If you must do it, put it at the end of your chapter, and don't make it a habit. You should be able to describe any important garment, and the clothing being worn should generally not be the focus of your narrative anyway. This same principle can be applied to extremely detailed, yet totally irrelevant descriptions of appearance/outfits.

9- Show, don't tell. With regards to the qualities of action and other information, convey that via description.
e.g.
BAD = "Thanks," I told him happily. (You are telling the reader the protagonist is happy.)
GOOD = I couldn't help smiling at him, "Thanks." (You are demonstrating/showing that the protagonist is happy)

Notes

I feel like I should make a polyvore picture of the pajamas I'm wearing right now just to show how ridiculous it is, but I don't even know how. Haha! :D Shame, you'll just have to imagine my sweatpants on your own.

(a) who actually wears pajamas? -a