I love world building - it's such an important part of making a fantasy/sci-fi setting believable and enrapturing. I'll share my three biggest world building musts:
1) Consider how geographical locations affect your world.
i.e if a location is southern and coastal, there's likely to be lots of fishing, sun tans, tourism, humidity, etc. The geographical location will affect not only weather, the type of people, and activities done there, but the natives' jobs, livelihoods, and traditions.
2) Pepper your world history with details
This will affect not only relationships between people, but also countries, and will dictate if there are ruins, new cities, large empty spaces, wildlife, etc. History for one location will be different from the history of others. One planet is made of cities and has a bad rep for shady doings. Another planet staged a rebellion against their government a hundred years ago, and the children of those leaders still hide in the wilderness. Little details are so, so important, and they make a world all the richer.
3) Kill your darlings
Sometimes, the coolest little detail you've come up with concerning your world will have absolutely nothing to do with your story. Sure, it's badass, and you love it, and you want people to know you've made it with your own two hands. But as far as relevance goes, it has none. Make the hard choice to cut it out of the final draft. It can still be part of your lore, but if you're cramming facts in there that have no significance to the story and won't come up again, cut it. It hurts, but cut it.
Those are what I've found really help me! Here's a link to my most meaty world-building story. It's a slightly futuristic fantasy featuring three major countries, each with very unique climates, governments, and cultures: