EXPAND AS QUICKLY AS POSSIBLE. I buy my first settler because I'll be busy building early wonders. By this time, you should've already researched iron working so go found a new city nearby. Ideally, you also want a new luxury resource so your citizens won't sulk because of the expansion.
At this point, you might already be getting attacked by an overwhelming force. Hold your fort with spearmen, an archer and maybe swordsman. Adopt Oligarchy for a +33% home turf advantage. If you tackled enough barbarians, great general is very close. I had one pop up just as Gandhi stormed me with his elephant archers. +45% GG and +33% oligarchy will give you the defense you need at this point.
If you have maritime states nearby, do NOT adopt other social policies than Oligarchy and Aristocracy. Save your stonehenge culture points for mid-game Patronage domination. Otherwise, use them on Honor, especially double experience.
If war is tough, go straight for Steel and upgrade to Longswordsmen.
After that, go straight for Machinery for Cho-Ku-Nos. This is when you finally get an edge and are able to stop defending and instead attack your neighbor. Raze their small cities, keep their capital, repopulate with your own settlers.
Hopefully, you conquered some very nice territories because now you'll have different priorities.
Mid-game
At this point, you're facing 1 or 2 other major competitors with a vast, technologically superior army. You can't take them on with your 5 chokunos. Set up a strong defense perimeter in the city nearest to them and hold fort. If terrain permits, create a chokepoint with a citadel or two, declare war on them and just keep defending. They will likely keep coming at you forever and your armies will get more experience and more GGs.
Always build your citadels on hills and always hold the higher ground so they can't bombard you from two squares away.
Your sole mission now is to get to artillery and infantry/mech infantry before them.
For this, you'll need a specialized science city. Could be your capital, but not always. I plan my science city wherever there's the most food, so that means farmed river+grassland tiles and ideally, a mountain nearby for Observatory. Research the techs needed for tech buildings, but don't bother with anything else.
Other cities should not bother with farms, instead TPs everywhere for money. Employ citizens in your paper makers in every single city for great scientists. Early, on, I personally like to build academies around my tech city, but you can also save them to insta-discover later. Expand your cities, keep happiness up, just get as many beakers and GSs as you possibly can.
Don't build buildings you don't need. You should only have one military city with barracks & armory, others can generate science after market/bank/papermaker/university/colosseum have been built.
If your competitor is warring any city states, ally them and gift them strong non-promoted units regularly.
I try to go for other wonders such as Himeji and Hagia Sofia, but I'm not always able to get them.
Late-game
Your competitor will be technologically superior for most of the game, but they didn't beeline Electronics like you did.
If you make it to mech inf. before your opponents: upgrade, attack and conquer. Tanks are crap, don't bother with combustion.
If not, you might need GDRs. I haven't made it that far yet.
That's it.