Creating a new world from scratch is a daunting task. It can be fun, I certainly enjoy it, but it also requires more effort than using an established setting. If done well, a new world can enrich your stories with wonder and depth. There are a lot of questions about this new world, and the writer must answer all of them. Preferably, before those questions come up in any story set in that world.
What is this world like? Is it an Earthlike planet? If not, we only have so many examples in the real world for what it might be like. There are hundreds if not thousands of worlds completely different from Earth in fiction. But if your characters are to be relatable, you have to have some way for readers to connect to this world.
Besides the similarities to Earth (or lack thereof), what makes it different? Are their people there? If so, how are they different from people in the real world? Is there an ecosystem, plants, animals, or other life unlike Earth? If there are people, what does their society look like? Are they more advanced, less advanced, or similar to some time in Earth’s history?
There are many more layers to fill out in building a new world. Social customs, religion, government and secular institutions all play a part in how your world operates (assuming it is peopled). Barren worlds can offer great settings too, especially if your story revolves around surviving hostile conditions. These concepts apply equally to fantasy and science fiction stories. Although sci-fi might allow you to use more technical language in your descriptions.
If some of that feels overwhelming, there are a lot of checklists, e-books, and guides available online to help you work through what your story needs. Keep that in mind too, you only have to invent what you need. You don’t have to spend a lot of money for many of these world building guides, either. It does take some time, but for me, that’s time well spent. Especially if it opens up new material to tell a whole world’s worth of stories.