thank you! <3 my biggest piece of advice is to start by writing something you find fun, and don't worry about appealing to anyone else other than yourself. the kind of idea where if you came across it in the wild you'd be giggling and kicking your legs because it makes you so happy. pure self-indulgence. i find i have the most fun when i'm writing purely to please myself and not thinking 'i wonder if [respected mutual] will like this...?' or 'is this too xyz to appeal to [shipper group]?' or 'do i actually have the skill to convey what i'm attempting to get across here?' it's fandom, it's meant to be fun, so don't put any undue pressure on yourself and just go wherever your heart takes you! your feelings for the characters and enthusiasm for the ideas are the most important thing, IMO. improvement will come naturally the more you write.

speaking of improvement, how-to-write guides can be useful but it's also easy to get bogged down in the details sometimes, and it's worth noting that guides are all written by humans with their own unique tastes and ideas that might not necessarily line up with yours, so don't take any one piece of advice as gospel. take what works for you and leave the rest. my main tip for improving on a technical level is to read, widely. a lot of aspects of a well-crafted scene can transfer across genres (for instance, one thing that i've seen discussed a lot is that sex scenes and fight scenes have a lot in common, since both rely on choreography, physical sensations, and emotional reactions), and the more you read the more you absorb different inspirations and learn what sorts of story structures, sentence constructions, characterization choices, etc work the best for you.

i hope this was somewhat helpful! good luck anon, i hope you have a blast writing your story!