Well, you can do that 2 ways. You can either capture an economic civilization and click economic when the game asks you if you want to. OR you have to capture then befriend other tribes in tribal...
You can't kill the people, because they don't matter anymore. You just have to attack all the buildings until they are destroyed. Once they are, attack the middle of the city and you'll soon take it...
Well, you have to be economic, and first you get the nation to like you by giving them nice comments and lots of money. Then you can propose a trade route. Wait a little while, and the computer will...
To begin with, build fast units(you can change your unit design later on) to take over all spice geysers. That gives you income boosts. Then change your unit design into balanced, or tough ones. Then...