It depends on why exactly i am making them up.
I have basically three pillars. Depending on my purposes, i may have more or less development in each of these.
* Geography. According to Germs, Guns, and Steel (and i believe it, though not quite to the degree the author does) the geography and available plants and animals have a huge role in what kind of society can develop. You won't find a great civilization on a tiny island with little food-- people who spend all their time avoiding starvation don't have the time or energy to build great civilizations. Navigable rivers knit areas together, while mountains divide... etc.
* Technology. This naturally has a huge impact. Weather a people can thrive in particular geography or not depends on their technology. Each major invention brings along it's own advantages and new problems. How the culture responds to these challenges involves the next
* Philosophy/Religion. This is where the culture takes what it's been given and decides what it should do with it. The cultural beliefs create a distinct personality.
And then if you start tracking these things over time, you have History.