Culture is one of the most important and distinctive parts of your nation. It includes things like food, language, social considerations, art expression, It will determine a lot of its historical actions, and in some cases may determine foreign policy as well. Cultures, in general, should follow some guidelines:
1: Cultures form around their environment. Cultures generally form dependent on their surroundings. For instance, a culture in a Europe-like climate may have historically had mythological explanations for the 4 seasons- but those in a savanna area will likely only have explanations for the rainy season and dry seasons. This is also true of geographical features- if your plot has a notable instance of a mountain, a river, or other features (like types of tree or animal) they may appear in your nation’s culture.
2: Cultures are influenced by other cultures. This may come in obvious things like loanwords, but it can also be more subtle. Maybe two cultures living in the same area grew to share some common values. Maybe they exchanged stories that were subsumed into each other’s mythos. Perhaps a religion was spread from one culture to another. There are many ways to have your culture influenced, so be willing to collaborate with other rpers in your area- that’s the entire point of roleplay anyways!
3: Culture influences history. This seems obvious, but it can have a very large impact on your history. For instance, a culture historically focused on single combat and strength will probably not have a great navy throughout most of history, and could have neglected its economy. Likewise, a culture based on navigating the seas would likely have been somewhat dependent on trade. Consider the facets of your culture, and how they would have affected your nation’s overall history.
4: Cultures change over time. Cultures are never static. Even over short periods of time, profound societal changes can happen, and this is true with overarching cultures as well. Consider how your history with your neighbors or internally made your culture adapt. Perhaps after consolidating your power, your nation went isolationist. Perhaps the opposite happened- an isolationist people become interested in empire-building. Pearhaps treatment of different sexes, ethnicities, and other groups changed over time. This is an important aspect of driving your history.
And finally, 5: Cultures are customization. These aren’t hard rules by any measure- they’re guidelines to help you. Many facets of culture (like language, for instance) change essentially arbitrarily, and you can customize the tenets of your culture to your liking. Just make everything semi-realistic, and try to adhere to the most necessary guidelines (it would be strange for a desert people to worship a winter god!) and you will have an interesting people whose stories will be told for millennia.