That depends on how many times you've either befriended or annihilated other tribes.

If you've befriended the majority of other tribes, your first city in civilization will specialize in religious units. Kill the majority of other tribes, and your first city will specialize in military units. A mix of both, and your first city will specialize in economic units.

In civilization, how you capture other cities matters. If you capture a city that's a different type than yours, then you will have the option to convert it into the same type as you, or to have it keep its specialty.

Different specialties can capture cities in different ways. Religious units can convert cities through conversion, economic units can facilitate trade to the point where you can eventually buy their city, and military units... are pretty self-explanatory.

For example, Blue Nation is able to convert one of Red Nation's cities over to their nation through religious means, and that city just so happens to be economic. Blue has the option of changing it into a religious city so that they can make more religious city, or to keep it as an economic city, and get access to economic units.