Creating a kingdom

So in my fanfiction, I'm creating a kingdom (have no idea what it's based on yet) and I need some help on how to fully structure and write a believable kingdom?
Can anyone give me tips on how to create and what I need to have?
for example; how would I make government, economic, citizens, monarchy etc.

Any help is appreciated!

Ok, there are two types of monarchies constitutional and absolute.
https://sites.google.com/site/monarchyrevival/types-of-monarchy
This site helps describe the two.
I would like to help more if you have any other questions.