Selecting A Genre
Not all worlds need to fit within a pre-existing genre, but many will. Even worldbuilders designing outside of common categories will find it useful to study or borrow elements from known world types. Genres simplify the building process, as they come with standard elements that can be incorporated as well as pre-existing audiences that enjoy them. Even those worldbuilders seeking to avoid typical tropes should familiarize themselves with genres to know what to subvert or to avoid.
Combining genres is common, and often certain genres, such as horror, find themselves nestled into a larger world type such as science fiction worlds or fantasy worlds. While genres can be a great place to start, they should be by no means constraining. Frequently worldbuilders will find inspiration in places they least expect, and borrowing elements from other genres can be a great technique.
If you have a sense of what type of world you wish to build, using the Worlds section of this site can be a great resource.

Do you want to build a world that contains magic? Or one with futuristic technologies?