First connect all of your cities via road/harbor, then improve tiles. Mine hills, mountains, and grassland with shield. Irrigate regular grassland, flood plains (flood plains + wheat + irrigate = 5 food! That's enough to feed four citizens and have surplus when only one has to work!) plains, desert.
Road/railroad everything. Road gives trade, which equals more money coming in, and faster research for techs. Railroad gives extra food/shields on tiles that already have them along with the commerce bonus. Later in the game, when all of your land is railroaded, pollution/invasion forces can be responded to in one turn.
As for workers, at first use your own workers for tile improvements. As you capture more and more workers, use them for improving and use your workers to make cities larger. (You don't have to pay for captured workers on a per turn basis, with your workers you do.)
Hope this helps,