just gonna ramble:
reading other people's fics is a great place to start. note what you like and what you don't. you can learn SO much from others
make the basic decisions like which characters will be in your story. will it just be pre-established characters (like a Steve x Natasha)? or will there be oc's or y/n?
come up with a good title, book cover, and story description. these are what are going to bring people to your story
rewatching the movies/shows is great for inspiration
https://marvelcinematicuniverse.fandom.com/wiki/ has been great for doing research and looking at timelines. learned tons from it
decide how much of the pre-established universe and events you are going to include in you story (for example everything up to Civil War happens so Infinity War and Endgame don't happen or maybe Endgame happens but Wandavision doesn't)
coming up with your own outline/timeline for your story is very important. it's the frame the rest of your story will be built around
be flexible! i have changed major elements of my story overtime and deleted other bits
don't rush! the story will start to tell itself at some point and you will just be the vessel typing it up
write what you know and what feels natural. if you're forcing it, the readers will know
if you have a general idea of what you want to happen, but aren't ready to write it all out yet put a description in brackets. i literally had [Doll, Bucky, and Steve live together in an apartment] at one point
you do not have to write scenes in order! if you just got inspired to write something that happens 20 chapters later then go ahead and write it!
write in something other than Wattpad! Google docs, Word, notes, anything
editing is IMPORTANT! i see fics on here all the time that have a great story but are painful to read because they never edited crap, smh
take your time! i started writing Not The Only One at the end of September 2020 and didn't post the first chapters until halfway into December 2020