Creating Realistic Cultures
Something that a lot of fantasy and science fiction writers gloss over, and sometimes in a critically offensive way, is why and how cultures come to exist.
Obviously the easiest way to figure out the answer to this and apply it to our worlds at large is to examine our own history. To simplify things, let's try to examine this from two perspectives: A Species-level and a State-level. Keep in mind that history and existence in general is complicated, so the most important thing is to think critically and do your research. This is just to get you thinking!
Find the basics. Sherlock Holmes once said that the simplest solution was usually the correct one. This applies to an extraordinary amount of things in life, not just Victorian-era mysteries. But, here, we are going to apply the basics to figure out why cultures exist at all.
What does your species need to survive?
If they're human (or basically human) then it's very simple. Food, water, shelter, clothing, sometimes energy. These things, these resources, have fueled every facet of civilization as it began across the face of our very own earth. Humans don't just prefer to have these things, they are essential for our bodies to continue living. As such, the nearby availability of those resources in question makes it possible to build villages or cities and then eventually large-scale nation states. We built these together so that cooperation could, at least in theory, make survival easier for the individual. A hunter-gatherer is going to live a more relaxed life in general, but a farmer that makes a surplus that his family can live off of can trade for items he needs but has no access to. This starts off with the essentials before branching out to desires.
So your species (however many) need to fulfill their basic needs, and then will likely seek out creature comforts. A city of humans (or human-like creatures) will absolutely have to begin on the banks of a river or source of fresh, clean water. That water will also give access to food (hunting grounds where animals come to drink, fishing options, and irrigation for farms). Water is also going to provide resources to build - perhaps wood from forests that spring up around the water to drink from it, or clay taken from the soil around it. That takes care of shelter. And then of course flora and fauna harvested by these people will yield material for clothing to protect from the elements. In more advanced civilizations it will also provide space for hydro-based energy. And you don't need modern technology like hydro-electric dams, it can be as simple as a water wheel mill to help farmers turn their grain to bread. After these needs are met, do people begin seeking out excess? Why or why not?
So you can apply this ideology in general to anything. Add or subtract needs and see what makes their existence more compelling and interesting. If your people have no needs then they likely don't have culture or civilization - but keep in mind that that's valid too! You just need to explore what happens in the vacuum. As an example, is your civilization composed of robots or automated sentients? Where did they come from? Do they have opinions, a hive-mind, a core processing unit? What exists for their reason to continue without the driving force of basic needs and greeds?
The big idea at this level is just the basics of why or how your people have begun to survive.
This is how it starts, the baby steps of your new culture. But a species of people who are creative enough to get these solutions is probably going to complicate things for better or worse. So start asking questions about how your new people continue on a day-to-day. What makes life worth living for your people?
What religion or philosophy do your people turn to for the big questions on life and death? Or even how the sun rises each day? Who is in charge of coordinating large-scale efforts? Are the two tethered together, wherein the leader is divinely backed? Is there a caste-system in place? Who makes the food, and who makes the tools to harvest that food? How do your people defend themselves, and more importantly what do they defend themselves from? Why or why not do they settle into cities?
At this point you're examining how they're using resources to take care of their needs and how their using the left-over resources to create art/architecture/technology/etc. And every culture, at least known to us, uses art. Often in part to help convey important traditions that have developed. Ideas are conveyed from one generation to the next this way.
Just as with our prior, species-level building block, remember that cultures solve problems. Here, rather than dealing with how survival is attained, we're looking at how the balance between that survival and comfort or new discovery. In our own world, there is a prevalent theory that culture evolved in some areas almost exclusively to gain easier access to alcohol! Ancient Babylon structured much of its culture around its temples; letting people communally store grain and resources in a temple complex. Priests and Kings became important because they were the ones who were responsible for ensuring that everyone had access to these resources - and had access to them in times of hardship. As such, Babylon put a high importance on its priesthood, and their kings were divinely associated.
Culture and society tends to build a large focus on how they share ideas amongst themselves or project their ideas onto others. So think about what they're sharing. Are your people focused on conveying military or physical might? Think Sparta, the Mongols, Sengoku Jidai era Japan. Do your people focus instead on trade and the accumulation of resources? Look at Ancient Phoenicia, or its infamous successor Carthage. Most of Renaissance Italy put a premium on trade, as did Southeast Asia's Majapahit. Religion, military, money, something else entirely perhaps. All of these things build traditions on how the society at large answers questions amongst themselves.
What traditions does your society have, and where did they come from? What festivals and religious holidays do they observe, and what ideals do they back up that the people in power in this society support? Are there things that people celebrate, or even worship, that their respective government does not support? In many Greek city-states, the worship of Dionysus was forbidden because the supported ideals in his cult included taking power away from the ruling class. How do counter-culture movements begin and how does society respond to them?
These are all just examples, and you need to examine what makes sense or what doesn't to you when you start building a culture. Remember that your people do not exist in a vacuum and that their respective response to resources and resource management, and how the extra resources are used artistically, will make a more believable and rich culture for your stories.
A WARNING ON THE WORD 'PRIMITIVE'
A culture does not need a state, but a state generally needs a culture. There is a misconception, left over from the colonial scrambles of the 1700's and 1800's, that technology generally defines how civilized or primitive a culture is. In fact as a word of warning, describing a people as 'primitive' is a good way to sound incredibly ignorant and piss people off. When you use your own culture, your own understanding of the world, as a moral bar that another culture needs to reach before they're considered your equal, then you're not just being racist but you're also just... incorrect. Horrifically stupid, in fact. Remember that that idea is used almost exclusively by colonial or conquering powers as a propaganda tool to describe their enemy as somehow 'less than' - thus making it easier to combat them or justify conquest. Shockingly, the same two solutions to a problem rarely work everywhere, and it's a matter of choice how people agree to behave.
Think critically, folks. Don't be a dick.