When you’re designing a world, there are bound to be different kinds of people and cultures. People usually wear clothes, and each group of people is going to have their own unique spin on what they traditionally wear. There are a lot of factors that influence this, and it’s important to consider why they wear what they do, and not just what they wear in the first place.
The first thing I think of when I think about what kind of clothes the people in my story will wear is the climate. If your characters are wearing things that wouldn’t make sense in the climate they live in, it’s going to be harder for your reader to suspend belief. Your reader is probably going to be puzzled if your desert-dwelling people wear parkas all the time, unless the desert happens to be a very cold one. Basically, think of what clothes you would have to wear if you walked outside in a particular part of your world. If you would put on a coat, it’s likely that the people who live there would as well.
If your species aren’t human, you should also think about their physiology. If they’re a cold-blooded species and can’t make their own heat, they may well have to wear parkas in a hot desert (at least at night, when the desert gets cold). A cold-blooded species also might not live in an area with cold weather, unless they had extreme protections. The same goes for a species that runs hot, living in an area with hot weather. Warm-blooded animals that live in hot areas generally have cooling mechanisms (sweating, big ears, panting, etc), so you should think about that, too.
Now that you have a general idea of what kind of clothes your people wear, you can expand on this. The type of clothes you just thought of while thinking about the climate are probably their casual or work clothes, or at least the general style of clothes. Here are some other kinds of clothes they might wear, if they’re relevant:
It’s also interesting to think about the aesthetic of your people’s clothing. What kind of colors do they wear? Pastels, neons, jewel tones? Where do they get the dyes for these colors? Why are these particular colors popular? Do your people wear patterns, solid colors, or both? What do these patterns look like? Are certain colors or patterns religiously significant? What colors do the military wear? Different ranks might have different colors. What are the colors of your particular country? Do people wear jewelry? If so, what kind, and why?
Sometimes the clothes people wear stems from traditions and taboos. Are there stories behind why your people wear what they wear? Who decided that certain groups of people should wear certain things? Do clothes differ by gender? Can certain genders wear things that other genders can’t? What kind of clothing is seen as inappropriate? What body parts should be shown and which should not? What are the consequences if someone wears something inappropriate?
This is just a starting point, but hopefully it’ll help you flesh out your world’s people and make them seem more real. Clothes are a big factor in differentiating between cultures and groups of people, and they'll stick in your readers’ minds. Best of luck!