Suggested Tactics
CITY MANAGEMENT
The first thing you'll need to think about when founding new cities is its placement. If there is a group of resources that will fit within a city radius, build a city there. Plus, cities built deep in the woods usually have great Shield production, but they will always be limited in population and finances. The best place to build a city is surrounded in grassland squares next to a body of water with a whale or fish resource within reach. Another resource that's great to have in a city's radius is a set of hills, hopefully with a coal resource in it. Because when you mine hills, it increases the shield production in that square to 3. Another good source of Shields is to have a forest square or two. Also, land improvements are a must around prosperous cities. Irrigate(and later upgrade to farmland) all grassland squares and mine all hill squares. Also, build roads whereever applicable so you'll get the +1 in trade. (+2 under republic & democracy) Rails will increase the Shield production in that square by 50%. You should especially build roads and later rails between your cities, so you can move your troops around your empire quickly in times of crises. And if you have a multi-continental empire, keep a troop carrier available. But, after the surrounding terrain is improved, you can further increase the rate of growth by building a Granary inside the city, which will double the cities rate of growth. Or if you don't want to worry about that, build the Pyramids Wonder which will simulate a Granary in every city. The problem with this is that the cities population may tend to grow faster than your ability to keep everyone happy.
Once you have a thriving city, begin building city improvements to help keep the citizens happy. A temple is recommended, but for a city that is just growing faster than your civilization can keep up, (not enough Luxeries) a coliseum never hurt anyone. But the main way of keeping everyone happy is by building happiness wonders. The Hanging Gardens is great for early on. J. S. Bach's Cathedral and Michelangelo's Chapel are some other favorites. But the Oracle is excellent if you already have Temples in most of your cities. But if you're playing at a higher level of difficulty, (King, etc.) or just have a city that refuses to be silenced, and if your current form of government is one that allows it, you can declare martial law by moving extra troops into the city to put down civil unrest. Also, in order to maintain a productive civilization and hopefully stay ahead of the AI players, you should build plenty of libraries and later universities to increase your science production and build marketplaces, banks, and stock markets to keep luxuries and tax revenue high. To speed along at science, keep the tax rate & luxuries rates as low as possible without unnecessary civil disorder and keep enough revenue to pay for all the city improvements. But what ever you do, keep a watch on your city improvements costs. A Coliseum costs 4 gold per turn, which gets VERY expensive if you build many of them. A Temple only costs 1 gold, and it is unlikely that you will need more than that except in cities like your capital which reaches size 12 quickly. But cities this large usually produce enough taxes to pay for it.
Another reason to strive for happy citizens is to gain the "We love the ____ day" status. The benefits of this status varies with the government you are under.