uwaaaa ty for da support, get ready for a bloated answer lol

generally I prioritize whatever I wanna see first or whatever idea paints the clearest picture. these ideas usually come from various fanarts, other stories I see/read (fics, novels, anime, manga) or when I'm having deep thoughts in the shower or before I sleep lol
for beginners, the important thing is to write as much as you can whenever possible in your schedule and observe different styles of writing: practice and exposure is important. A good way to get short+easy practice that I enjoy is finding art that clicks and then writing a short scene around it since the concept is visualized for you (if you are tempted to post these practices then be considerate of the artist ofc).
don't be afraid of having a rough rough rough draft for your stories, whether it's just dialogue or you're jumping between sections. as you revisit it, you come in with a refreshed perspective of your own idea and can adjust it gradually. it's ok to waver on concepts over time too through this, it depends on how much you care about the idea or concept you have.