Before you can tax anyone, the money must first exist. There has to be an economy in place.
First, you have an territory with a particular location, climate and geography. Next, you have in your territory a specific number of people of a particular ethnicity, demographic and culture. Start with the basics. Google Earth can help locate your fictional country. Once you have the place, you'll know the resources, settlement, energy and transportation options.
Then, you have to invent or borrow a history for that nation. Was it always a sovereign nation, or is it and emancipated colony - if the latter, by what means was independence gained? Or is it uninhabited and you're leading a diaspora to settle it?
Have you considered the form of government? You need a constitution to lay out the principles on which the country is governed.
If you really want to do this thought-project, think like a world-builder.