It depends on the situation. But I can give some general tips:
1)You generally don't need an extra worker when you start. The starting worker will do. He should improve the capital or the main settler/worker producing city as fast as possible as this city will determine the speed of your expansion.
2)If a city has a large food surplus, you should consider building a granery. It will add more "surplus food/growth" to your empire than another city will. Building a granery in a city with 5 food surplus will make that city grow as if it had 10 food surplus. Another city will probably give you 2 food surplus. Maximise empire-wide food surplus to maximise your expansionrate.
3)If settlers take a long time to reach the frontier of your empire, than you know you have build too few workers. In flat terrain workers can speed up your rate of expansion because they triple the movement rate of your settlers and roads can be build very fast in flat terrain.
4)Improve your critical terrain squares first. The terrain squares that have a base food output of 3 or more should be irrigated as fast as possible. Although despotism reduces the food output of these tiles by 1, irrigating will again increase it by 1. This will thus give you a terrain square that produces 3 or more food. These are the squares that make settler and worker factories possible.
5)First expand, then build buildings. There will be time to improve your empire later. There will not be another chance to claim unclaimed territory. You also don't need veteran troops to defend those first cities. The first troops are only present to decrease unhappiness and to discourage the AI to attack you. Sometimes of course buildings can be usefull. An example is a temple that will enable you to use a tile that has a base food output of 3 or more (especially when you're religious)
6)Learn to use the luxury slider (in F1-menu use money to entertain your citizens) in stead of entertainers to reduce the unhappiness of your citizens. If you use entertainers, it will reduce your food surplus and thus your growth.
7)After the expansion-phase, you should build as much workers as needed to make all of your citizens in each city use totally improved tiles. An improved tile is a lot better than an unimproved tile. 1 road build by a worker will already pay for the upkeep of that worker. If you cannot improve any tiles anymore, you can add the workers to cities or use them to build a worker consuming terrainimprovement. If you are a warmonger, then you will always have terrain that needs improvement in conquered territory.
8)Slave workers cost no upkeep. Use them whenever you can. Never disband them, never add them to cities. Free labor is always better than paid labor.
9)When cities cannot grow anymore (aquaduct or hospital is needed), and you don't need anymore workers, then you should reimprove some irrigated tiles to mined tiles to increase production.
10)In order to improve the right terrain squares, you should not automate your workers at the start of a game.
All of the above are just guidelines and should NEVER be used as absolute rules.
A settlerfactory is a city that has a very high amount of surplus food. You need tiles that have a base food output of 3 or more for this. A granery enhances the growth of these cities a lot. The term settler factory is used mainly for cities that have a food surplus of 5 and a granery. This is the ideal situation. In this situation, one can build settlers at a rate of 1 every 4 turns. See also these two War Academy articles:
Tutorial: Babylon's Deity Settlers
and
Cracker's Civ3 Opening Plays Site
