Since you have already been given a lot of good advice, I will try to build on it by explaining in detail a specific strategy that I have used with great success on the deity level. You might find it helpful to try the strat as I have laid it out or to pick out a few ideas from it and apply them to your own strat. You should learn the more standard startup (warrior, warrior, galley, etc.) but don't be afraid to deviate from it. I use a research-first strategy in which I delay getting barbarian rewards and possibly lose a lot of them to the AI, but in exchange I get some great first-to-research bonuses. Only the first few steps are unique to this strategy, but I have provided the steps needed to keep the advantages this strat should help you gain. I know you are not a total noob so you may already know a lot of this but anyway, here is my strat:
1. When you found your first city, assign both workers to sea squares, and set research to bronze working.
2. When you get a third worker put the in the forest, building a temple.
(note: if you are not Japanese your population will not increase while you are working only sea squares, unless you are Indian and have the right resources)
3. Research iron working next. You should get bronze in 5 turns and iron in 7 more. The AI will not have iron yet, so you get a free legion.
4. Switch production to legions and send your free legion out to (finally) explore.
5. Study alphabet, then writing. The AI never seems to study writing, so you will get a free spy that you know is the only spy in the game.
6. Hopefully you will get your free settler during this time. Start building legions in your second city. You should not need to rush a barracks, since your first few legions should be veterans from fighting barbarians. It might be a good idea to rush some legions, although you will have to come up with some money for roads later. If you havent gotten your settler yet make sure you really, really need whatever you are rushing.
7. Once you have 6 legions and a spy, send them to an enemy capital. If you are new to this and not sure if you built your armies soon enough, feel free to save the game before declaring war. If you know what you are doing and just hate to lose I guess you can still save. Have the spy take out fortifications to give your legions a better chance. You are likely to lose your first army, but your second one should win. You will probably have to spend a few turns after the initial strike eliminating single archers, but the AI shouldn't have any more armies.
8. Let your armies heal if they need it, and move on to the next city. If the civ whose capital you took has another city, that is your target. Since you don't have a spy it will be difficult for you to continue taking capitals unless they are seriously underdefended. You should avoid capitals and cities with walls until you have knights or catapults.
9. If you receive an offer of peace you should accept it unless it prevents you from taking a city immediately. Multiple civs will probably have declared war on you by now and you will have to spend some time killing off their legion and archer armies.
10. Once you have taken a few cities you should switch production from legions to archers. There are two reasons for this: you need to defend your new cities and you will have better offensive units soon. You should be working towards mathematics or, better yet, feudalism. A third potential reason to build archers now is that they will be useful much longer than the legions will. With a few defensive upgrades they can defend your cities for a long time; with march they can escort knights or catapults.
11. If you get a great general, be very careful with the unit, but try to keep upgrading it with easy wins. If you can get the march upgrade you can bring the unit with your knights and give them all a nice bonus.
12. By taking a few cities and hopefully wiping out at least one civ, you can buy yourself some time. Use that time to build a library in at least your best research city and a barracks in your best production city. It is also a good idea to build a temple in your most populous city so you can get more great people sooner. Your knights will be much more effective if you can rush feudalism or the samurai castle.
13. Once you have a few armies of knights, send them out with anything that can march. Your target might be the weakest civ or the one that is giving you the most trouble, but it should be someone nearby. If your target civ is still using archers you can attack their capital and any cities with walls; if they are using pikemen you should take their easier cities while you start making spies. The AI will often have spies at this stage but rarely spy rings.
14. The next tech you want after feudalism is currency: you will want to put markets in a few gold-making cities and you want to make sure you get banking when you get 250 gold.
15. You will also need navigation. You will want a couple of galleon fleets to sink all of the annoying galleons leaving troops on your shores. Once you have your beaches under control you can use your galleons to give your knights support and to sink any new ships the AI makes. You should also have your eyes on steam power for much better support.
16. If you have enough bonuses (samurai castle, great general) and make use of spies and naval support you should be able to take cities even if they are defended by riflemen.
17. Eventually though, you will want to turn them into tanks. Figure out how many turns of research you need to get combustion, and time your building of Leonardos Workshop accordingly. If you are keeping up with the tech pace you might get to build the workshop, but you might not. If you are behind on tech you might want to build it right away so the AI cant use it. At least you will get to turn your old archers into riflemen.
18. By the time I have tanks I am usually more interested in getting cultural flips or building spaceships. I always try to build the Manhattan Project as soon as I can so the AI will lay off for a while. If you want to go for domination, you might be able to do it with tanks, provided you have battleships supporting and plenty of spies, but you might need to resort to artillery or bombers. Tanks are better suited for killing units the AI sends into your territory.
