Sadly enough the gameplay kind of has it's manual, generally people don't have to settle as much cities as the AI because we tend to only select the locations that will produce a good city, what's also important is to realize how important warfare is and how to do it, unless you have a specific UU you want to abuse generally try to go for a mix of units that should contains siege weaponry and whatever units you can defend them with best (I personally like to have around 3-5 siege weapons that I keep alive during the game so it get's insane promotions then a couple of swordman or the relevant era's equivelant a couple of archers to take down advancing units quickly).
Eventually you will very likely have more cities then the AI, but not because you built them.
What's also very important when conquering a city is to see how useful it will be, look at what buildings it has, it's position and if it looks bad just burn the place, if your not gonna use it it's probably just gonna be a burden.
Your opponent will pretty much always have more warring units then you, the only way to deal with this is superior tactics, use your siege weaponry effectively, build units with alot of movement points to take theirs down. The AI is terrible at positioning and knowing what the terrain values means so often times you can just select a good bottleneck they will have to pass, deplay your siege weaponry and just murder them while they try to reach your units, you can also turn your Great Generals into Citadels that are absolutely amazing at slaughtering units, even if they stand on the citadel they will still lose 30% of their hitpoints per turn and any unit that is adjascent to it, so perfect if you have to defend a more open position.
It's pretty simple really, the AI will always cheat but it will make up for it by being terrible at pretty much everything, exploit it by warring, rushing tech, rushing wonders (especially happiness and science boosting wonders are very important, Notre Dame, Porcelain Tower) and generally not making bad decisions (like make buildings that just drain money but hardly do anything), and if you really have no idea what to do just check some of the LP's that people have alluded to earlier.