Like @cassandraia said, I’d recommend looking at real-world government types. After you’re sort of familiar with how they work, you can even adapt them. For instance, there are theocracies (nations run by the religious leaders), but what if instead of religion it was science that was most important and you had a technocracy? Or what if the nation was run by its greatest artists?
The values a nation was founded on will influence the type of government it has. Take the U.S. as an example: because we were built on values of freedom and designed to escape tyranny, our government was originally built to give the people say in what goes on. And don’t be afraid to mix government types; the U.S. is a democratic republic, not purely a republic or a democracy.
Another thing to keep in mind is that governments change and/or degrade over time, so the age of your nation will make some difference to its government.
Fantasy/dystopian/sci-fi author. Mythology nerd. ENFP. Singer.