As far as I'm concerned, the name of the game is being ruthless and trying to take over as much land/resources as possible as quickly as possible.
A general early game strategy I've found to work *up to prince level* on a consistent basis on small to huge maps:
1. Concentrate on workers, settlers, archers/warriors early game. Maybe parthenon and pyramids.Try and build about 4 or 5 cities early. Get as many resources as you can as quickly as you can. Civs in your way, or surrounding a city you built to catch a resource, you will kill shortly. Basically what I'm saying here is don't let defensive worries stop you from building a city in a prime spot. Your goal is to be on the offensive.
2. Your early war should look something like this: Get some copper, and build tons of axemen and take over as many cities as you can until the AI develops harder to beat units or you feel you've adequately expanded for the time being. The key is here is *quickly.* Don't sit around building granaries and libraries and temples and monasteries early, just build lots and lots of units and maybe a barracks or a library or a granary when you have a chance. In general build lots of units as quickly as you can. As your army gets larger and larger you can take little breaks in building units and build improvements.
You should go for catapults early. Once you get some catapults then you should be *unstoppable*. Keep building *lots* of them (I mean
lots of them) and don't worry about suicide missions, that's what they are here for. If you have enough catapults and you play your cards right you can keep the same stack of axemen/swordsmen/horsearchers/whatever alive for a long, long time, because the catapults are doing the "heavy lifing," i.e. weakening the defensive units so you just waltz right in to their cities, all the while developing much needed experience. Lots of catapults and lots of any offensive unit = free continent for you to play with. Be aggressive, even if you are not the aggressive trait. If you've got a special unit like praetorians then even better. Horse archers take some time to develop but are also good for this. If the city you are taking over isn't on a good location then raze it. Don't worry about making early neighbours mad because your goal is to eventually eliminate them, but don't fight many people at once early on. Pick people off one at a time. Appease the others and then, when you're done with one of them, go after the next one. Rinse. Wipe hands on pants. Repeat. Once you've got the continent to yourself, sit back and enjoy your massive wealth and build to your hearts content. At this point any win scenario should be open to you. If you're good, you got the whole continent to yourself before they even met any of the other civs, so they wont be mad at you for "declaring war on their friend" or any other stupid nonsense the AI gets mad at you for.
3. Dont worry about being at 50% for a while. Cities that are not either GP cities or production cities should focus on being commerce heavy, with as many cottages as your food and health allow. Eventually when they become villages and towns you will be raking in the dough and will catch up. If you're at war with the AI they go a little slower tech wise anyways.
4. Your early techs depend on what you start out with. Don't build workers until you've got something for them to do, but you should have something for them to do early. The wheel, mining, agriculture and bronze working are "must haves" early. Bronze working allows you copper and axemen, which you should build lots of early and kill kill kill. If you've got a resource that requires a specific tech, do that tech first.
5. Don't found religions, take them over. You should be at war early anyways and if you're lucky the AI has already built the shrine and spread it anyways. Founding religions waste valuable time spent elsewhere, and when I say elsewhere I mean *kicking the AI's butt*.
This is just one strat out of many, but for me it works almost every time. On monarch this has worked as well but the AI is more unpredictable, they might show up on your weak side with a fairly large army if you're not careful. But, I have been succesfull with this on monarch. On tiny and small maps obviously the goal is to just kill everyone as quickly as possible so I guess this strat works there as well, I've only played one small terra map (meaning the AI is all on your continent so a conquest win is easier) with the Romans on noble and this is essentially what I did, winning around 990AD.
Also dont take this as the be and end all strategy, I have overlooked many things here and it is impossible really to briefly "sum up" what you should and should not do. To recap, the AI is a scourge that needs to be dealt with severely and with utmost prejudice, unless they are giving you stuff you need. Then, they only need to be dealt with severely and with utmost prejudice when you don't need their stuff anymore.