@Popupduckling
1. So for prompts it's a mix of things. A lot of times, I'll read or watch something, and that'll inspire me to twist the concept into something of my own. (ex. Mark of the Firefly was inspired by one of my fave middle-school books, Mark of the Dragonfly, and I just bastardized it until it became my own idea.) Other times, like in my Hawks and Tokoyami series, I'll think of things that fit the theme (in this case, things birds do) and how it would be fun to see how the characters deal. It's also totally ok to use writing prompts, or make your own spin on a preexisting fanfic tropes, nothing in life is original so don't feel like you have to fit into some concept of perfect originality; if it's been done before that just means it was a good idea.
2. Depends what site you're on. I'm no wattpad expert, but they seem less important on here than on other fic sites. I'd say that on here the title and cover are more important; my fics got more traction once I listed the ship in the title. On a site like fanfiction.net there are very limited tags to use, so it is important to choose what you think best fits the story, or else it might become unfindable to people who would want to read it. On Ao3 (my beloved) tagging is v important. You can tag every trigger, trope, and fetish imaginable, and you should tag the important stuff that applies to your works, people find fics on ao3 by using tags, so good tagging really helps!
3. I get writing slumps pretty often tbh, and I've found the best thing to do for me is not force yourself to write, and instead consume some new media (not fanfiction, actual other shows, books, and movies). This is what helps me respark my inspiration, because I'll see a cool concept that seems fun to write, or I'll feel compelled to write for a new fandom. Sometimes a writing slump is a larger issue of burnout or dying interest in a fandom.