If you desire to add sentient, living creatures to your planets, you should realize that most beings like us live together amongst one another. e.g. A herd of Buffalo, a murder of crows.
You should also understand that a civilization is different from an Empire or a Kingdom. A civilization is, by definition, the stage of human (or other living creatures) social development and organization which is considered most advanced. This means that a civilization can only contain one species, which is important.
An Empire is an extensive group of states or countries ruled over by a single monarch, an oligarchy, or a sovereign state. e.g. The British Empire.
A Kingdom is a country, state, or territory ruled by a king or queen. e.g. The United Kingdom.
An example of a civilization would be a tribe of cannibals ruled by a Shaman. It is not classed as an Empire because it is not a collective of states, and it is not classed as a Kingdom because there is no heir or monarch.
In Science Fiction, an Empire is a common addition, but Kingdom not so much. Usually, an Empire is a group of planets or systems ruled by an Emperor or a monarch. A Civilization is often situated on one planet and is smaller or more insignificant than the interplanetary rule.
Sometimes, you can create an Empire by combining several civilizations on different (or the same) planets to come together united. e.g. The Roman Empire consisted of many different civilizations which had been taken.
A civilization could also be a collection of minor Empires or Nations on one planet. e.g. The Human Civilization on Earth.
Often, a civilization could be a city or a town. You could create a planet covered in city and divide it into sectors. Each sector could be a civilization.
Civilizations, overall, are not that important (to me anyway), but I thought I would cover them before Governments or Nations because I think it is important to be able to distinguish one from another.
While we’re on the subject, the next post will be about (in more detail!) Nations.
I will probably do Governments or Monarchies after Nations but I will not be doing Aliens. This is mainly because I want to save them for later as I know it will be a very long posts (or posts). This is also because I feel that the creatures in your universe should be your idea, and no-one else’s. I don’t want to have to tell you what to do, so I will be covering them, just not now.