Before starting a game, make sure preferences is set to "always wait at end of the turn."
Start a pangea standard size map with 70% water, wet, temperate, 5 billion years old, no barbs.
First build 2 or 3 warriors to explore the map and make contact with as many other civ as soon as possible.
Then build a settler or granary and continue building settlers and settle as many cities as possible. Settle the cities about 2 tiles apart (city-tile-tile-city) but not at all cost. Varying the distance a bit is OK.
Let your cities build more workers and more settlers, with warriors in between in the event the city fills the production box faster than is grows. (since workers and settlers cost population)
Aim for about 1.5 times as many workers as you have cities. and use them to road every tile that is worked by a citizen first, and then just every tile. Improve easy to improve tiles first, and more difficult ones later.
Do not build wonders, do not build city improvements. Except for a granary in your capital, and maybe an aquaduct in your productive cities if they need one.
Once most of the freely available land is claimed (either by you or the other civ) build barracks in productive cities and start building fast moving offensive units (horseman) in those barrack cities and conquer one AI civ at a time. (at this point, you can start disbanding the regular warriors you've build during the expansion faze, they've served their purpose)
Keep units together in stacks (12 horseman in 1 stack for example) and attack enemy cities with all units in the stack at the same turn, do not send them in a few at a time. If you have losses, wait for replacements to arrive from the back before moving on. Do not stop building more units while you are at war.
Continue to also build settlers and workers to quickly fill up teh land you took from the AI, you can build them in the 90% corrupt towns far away from your capital, or build them in a city that is dedicated to pumping out settlers and workers.
Tech wise: Invent alphabet > writing > Code of law > philosophy > republic.
Then trade the more expensive tech you have for the cheap tech you skipped with the other civ (this is what early contact is important for) then go on and continue researching beyond what the AI have.
Once you've got this down, you can try to do better in your next game by building a couple of city improvements or even a wonder in a city here and there.
Return of investment is the keyword.
