World building is similar to creating characters but more complex. If you're anything like me, this means it takes a while longer to create a believable world than it does to create a believable character. The more you work at this world, the more detail you can go into. The more detail you go into, the more people will want to live there. And that's exactly what you want. You think Harry Potter would've been so successful if JK Rowling didn't know the wizarding world inside out?
You need to think of everything, but here are a few things that are essential: What's different from our world? What's the same? What's important to the people who live there? What do they hate? What do they love? How is society run? How is the class system set up? What are the cultural norms? What is the collective aim? What sort of education do the people have? What sort of jobs do they do? What are the roles of the family?
The structure of your world is the basic form you can work off to create everything else.
The details will likely come to you in rushes and drips, impossible to predict. In order to speed up, you can live in your world. Immerse yourself everywhere you go, and the details will start flowing. Whenever I'm bored, I do this. I'm usually walking somewhere or in the car.
Basing things off the real world is fine too. While I was writing the beginning of Fire and Ice, I was on holiday, and I used the complex we were staying on to shape the world of Shards and Flares. The very basic structure of the buildings let me breathe life to the Training Centre. Sometimes starting with a blank page is too scary. Letting your real world influence you is a powerful thing.
Once you're in and you know your world inside out, you need to be aware that the reader doesn't need to know everything you do. They need to know the basics and what's important to your story, but don't bombard them with detail.
World building takes time and patience, but you'll be infinitely glad that you took the time once everyone's wishing to be a part of your world. *cue The Little Mermaid music*