World building is a fun place to be. You get to create something out of nothing and have your characters live and breathe in this fictional setting that's come straight from your imagination. Just make sure you stay consistent--you want to keep your reader enthralled by the world in which they find themselves, and consistency is key.
Your world doesn't have to be in outer space or on a different planet or even in some hidden corner of our own planet earth. World building means creating the solid setting in which your story takes place. It's the time period and the place where everything happens. It's the geographical markers and the weather and the physical properties that are normal for the world in which everything takes place. If your story takes place in an office building, it's the cubicles and the offices, the carpet and the air conditioning, the elevator and the stairways and every one of the office workers and custodial staff and even what your characters eat.
The best part is that you, the author, get to make the rules. You're God in the world of your novel. You decide what is what. If there's no gravity in your world, everything floats. If there are werewolves or vampires, aliens from outer space with ten eyes and appendages that have adapted to the necessities of their world, it's your call. No water? No problem. Your characters live by sucking on ether? Cool. Everyone can fly through solid objects? Wonderful.
The only advice I have to give on world building is that you have to be consistent in terms of the rules you've set for your world. I'm not saying you can't alter it to advance your needs--you can. But you have to have a logical (in your world) explanation for why something that's been one way for the entire novel is suddenly able to change.
Now go on and create your world. It's waiting for you to bring it to life. And so are you readers. Write on, friend.