U never played the CIV1 or CIV2? Coz then there is a bit much to explain.. but here goes anywayz:
Under Despotism, 2 units in a town, will keep people happy untill city gets like size 6 or so. You can see that in the townwindow (zoom to [townname]), coz the units got a little smiley attached to them. Other forms of government gives different number of units that make people happy. Temples are IMPORTANT - both to keep peeps happy and to expand your culture. Collosseum also, but its much more expensive...
If people are too angry, you will get a riot. If you go to citywindow in a town where there is a riot and you click on the city on the minimap, there will automatically be assigned enough entertainers to keep people happy. But you'll get riots once in a while as long as your cities grow.
If you press F1, you will come to a window where you can adjust taxlevels (I almost always have 0% entertainment, 50% or higher science). The percentage U don't assign to science or entertainment is what will be incomming money.
AI swap techs like madmen and you CANNOT get in front with techs in CIV3 (that's my experience anyways). Just accept it and trade luxuries, money and resources for techs instead. In fact, I usually don't spent much taxes on science in the first period of time, bcoz you can buy the techs from the AI instead. A tech that might take you 40 turns to research, you can buy from AI for somewhere around 50-100 gold... So buying techs is in fact much faster than researching them yourself...
Hope this helps...