i recently joined a site called Harry Potter Fan fiction and when you write a story you can make a banner but i have no ida what program to use to make the banner
Well, most people use a program like Paint Shop Pro, Photoshop, or GIMP. Also, if you have no experience photo editing, you might just want to join the-dark-arts.net, which is where most HPFF authors get their banners. You just join the website and post a banner request, and an artist makes you one.
Also, if you’re looking for good stories, I suggest The Art of Breathing and Atypical.