Chances are your world is going to have a wide variety of different cultures. Because of this it may be a good idea for you to create at least a few different cultural groups in your world. Of course it is entirely possible that the main culture in your story is isolated because of geography or society and you will never need to show other groups. However it is still a good idea to create other cultural groups because they may inspire a different story within your world.
You can base the cultures in your world on cultures in real life, but be careful when you do. You don’t want to misrepresent real world cultures and that is a very easy thing to do. You don’t need to use real world cultures however. Instead you can create them from scratch. Human culture is made up of traditions, family structure, language, religion, rituals, song, dance, and viewpoints of different natural phenomenon, as well as a few other things. While you may not need to include all these things in your story, you may want to go the extra mile and go in depth with your new culture. But you don’t need to do all this and you can just scratch the surface.
Now the last thing you should do is come up with the ways different cultures and cultural groups are seen within other cultural groups because if you want your world to be realistic you need to include racism and cultural discrimination when creating your world. Most major cultural groups within history saw themselves as superior to everyone else and this creates conflict between this cultural group and other ones. Do not ignore this aspect of worldbuilding just because its a difficult thing to write about.