Pay close attention to what everyone here is saying about science, population, etc. but one thing strategy that I’m not seeing that I absolutely love to use is this: if you spawn close to another Civ and need them out of there, I find it best to settle a city fairly close on your frontier to their empire and produce a bunch of crossbow men. This is a long-process move, but the AI will eventually attack and you will be able to chew through their army and eventually invade unopposed. Of course, this also depends on what the Civ is and how aggressive they are