Like it or not (and I certainly don’t) economics is an important force in any society. It can be a great place to start building your world because it’s based around values, desires, and ways of life. Give something a value, and you give your protagonist a reason to want it. That’s a story in and of itself.
That said, I don’t think economics is as important as a lot of people try to make it. No matter what crazy kind of financial system you come up with, if you can answer the following questions, you’ll have your economics down and will be able to move on to more interesting things.
What resource has value in your world?
Why is it valuable?
How is this resource obtained?
Where does it come from?
What jobs are involved in procuring it?
Who has large quantities of this resource already? How did they get it?
How is it exchanged? How can someone acquired more, physically or symbolically? What sort of money system does your world have, and how does it relate to the valuable resource?
Real World Example:
The following is oversimplified for the sake of argument: gold has value because people like it, but it’s scarce enough that not everyone can own it. It comes from the earth and is obtained through mining; different methods of mining require different kinds and numbers of works. For the most part, the ruling class and federal governments own the most gold. It can be exchanged in small quantities; but more usually paper money is exchanged in its place, which each note representing a given value of gold.
(That’s how the paper money system was originally envisioned; but you can go have an argument on the further development of that somewhere else on the internet.)
In Suzanne Collins’s Hunger Games trilogy, the Twelve Districts of Panem are each built around the development or acquirement of a particular resource. Coal is mined in District 12, textiles are created in District 8, and fish are harvested in District 4, etc. These resources are valuable because they support the lavish lifestyle of the Capitol who, in hording the nation’s resources and military power, are the clear ruling class. The life of each person living in the Districts is built around acquiring their given resource, because that is the way they acquire food for themselves and their families.
Part 2: Geography