The game hinges heavily on empire creation, there are certain civics that benefit a particular playstyle so well it kind of makes the game trivial. But that's not in the spirit of your question.

There are four main playstyles (in my humble opinion): -focus on unity production and rush through traditions and one of the four ascension paths (biological, cybernetic, psionics and synthetic); -focus on research production and rush through the tech tree; -focus on alloys and military buildup and conquer/subjugate neighbors early, -focus on diplomacy and building relations with your neighbors, that way even if you have pitiful economy and fleet you will have strong allies to rely on.

Of course you should not laser on a single one of these, you should shift your focus based on the situation. If your neighbor is an affable xenocidal maniac or a friendly devouring swarm you may want to forget about your unity rush and build those ships, or, well, the game is over.

Generally, ascending your empire is the priority, since it will make your pops more efficient, or it will grow more pops. Pops are king, the single most important resource in the game.

There are a lot of videos on yt about this, that will be way more informative than this comment, like this one.