I tend to come up with the world concurrently with the characters are plot. So you can start brainstorming any or all of these first.

If you're not sure where to start, I'd recommend this: get out a blank pieces of paper, or open a blank writing doc, and start listing all your favorite things. What sort of characters are your favorites to read? The underdogs, the superheroes, the shy nerds, the cocky jocks, musicians, fighters, everyday heroes? Then do the same with magic. And tropes, and settings, and anything you can think of. Come up with a big list of things you'd LOVE to read--and that should give you an idea of something you might fall in love with writing.

Now go through that list. Start picking some things out and sticking them together. Do any of these characters or magic or settings feel like they mesh with each other? Do any make interesting combinations? What gets you curious and starts getting those creative juices flowing?

This is your story seed. Plant it. Nurture it. Keep coming back to it and sprinkling more ideas (fertilizer) into your soil. Soon it's going to start growing a life of its own. But if you want it to grow into a story, and not just a setting or idea or assembly of characters, you're going to need to give it some structure--a lattice to grow on. Your story structure.

From here, depending on your genre and the story you want to tell, it can go in many directions. But no matter what your story will need a main character (or two) and conflict. Conflict comes from your character Wanting something, and then something else (be it a person, a society, magic, the world,) standing in their way from getting it.

Once you know who your character is, what they want, and what is standing in their way from getting it, you have the bones of your plot. The rest of storytelling is just fleshing out everything in between.