There are many ways to begin worldbuilding, and all ways are equally valid. You might have an idea for a history of a world, or you might have a hero or character in mind that you'd like to build a world around, or you might have simply started with a weird dream you'd like to evolve. Everyone begins differently, but this post will be an effort to show a way I like to begin, which I call starting by gestalt.
Gestalt is originally a German word borrowed into English meaning a unified whole. You might think of it as the physical theme of your world (separate from a moral theme or literary theme, which are more abstract).
The question you might ask yourself is: "what separates my world from all other worlds?" Or perhaps "What singular unique feature is a central focus of my world?"
Many great worlds have gestalts, while many other great worlds do not, so if you don't have one, you need not worry about trying to come up with one. Star Trek does not particularly have a gestalt, for instance. Indeed, oftentimes science fiction worlds or realistic worlds do not have nor need a gestalt, as they are an extension of life, which, one might say, does not particularly have a gestalt of its own. Fantasy can also be gestalt-less, as The Odyssey hardly has a unique feature that unifies and defines the story. But coming up with a gestalt can serve as one form of springboard to get your worldbuilding creativity going.
Sometimes the gestalt is simply the ultimate goal of a quest; in Tolkien's Lord of the Rings, the gestalt is the one ring: the magical macguffin around which the entire story revolves. The gestalt of Stephen King's Dark Tower series is the titular tower, to which Roland and company are always in search of.
Othertimes, the gestalt is the very geography of the world. One example (which I am sorry to have forgotten the name of) is that of a world whose inhabitants live on the face of a sundial. Perhaps a particular unique feature of the landscape is ever-present, such as Frank L. Baum's yellow brick road of Oz.
In a similar vein, the gestalt of a world might be the method of travel. The MYST series centers around the idea of transporting between worlds through books. In another comment I discussed how one of my worlds uses waterrises to travel between worlds.
The gestalt of your world can work as a distinct hook to lure readers or players into your world, and want to explore. Having such a symbol of your world makes it inherently memorable and easily recognizable. Few people are likely to confuse the One Ring with a ring from another story, and surely no one has any other name in mind but Oz when they hear of a yellow brick road.
Does your world have a gestalt? If so, can you describe it? Perhaps, are you debating between a number of gestalts? (I find this to be my problem often.) Have you ever started a world by gestalt, only to have it disappear as the world evolved? Are you looking for a gestalt for your world to spice things up? Are you considering one now that you've read this? If you don't have one, and you're stuck on how to expand your world, you might talk about what you currently have, and ask about avenues for adding a gestalt. It's never too late to add one to create the pop you're hoping for, and it can take your world into a brand new and completely unique direction!