The most important element of your city and the main resource are people. When there are not enough people, the economy will experience shortages and companies will go bankrupt due to shortage of workers. However, if you have too many people, high unemployment rate will lead to a large dissatisfaction.
The population is divided into four social categories: workers, skilled workers, executives and elites. It is a very conventional division because not every company needs executives and not all executives work in managerial positions.
Workers at the beginning are a very large group - they work on farms and in the industry as well as sellers in stores. However, when the city's economy starts to change into a more modern one, their number reaches a steady, stable level with small fluctuations generally related to the size of the heavy industry and manufacturing. They have moderate needs.
Skilled workers constitute a middle class - they work generally in offices and as managers in factories. But when elements of modern industry appear, they will be employed as ordinary workers en masse and their number will prevail unskilled workers. In addition, the majority of public buildings require them as employees. Their needs can be a bit more problematic.
Executives are upper class. At the beginning, they work only on high positions in factories and offices but in the modern industry they will do a regular honest work. Virtually all high-density buildings need them. They require many amenities - a large number of shops and entertainment centers.
The Elites are cream of society. They don't engage in the dirty work, they are presidents, vice-presidents, chefs, ministers and their secretaries. Advanced areas and special buildings require them. To meet all their needs is almost impossible.
You have to be constantly taking a look at Demographic statistics on the toolbar. It will show you the employment rate for the number of people available to work. It will also show you satisfaction of different domains - entertainment, trade, labor and taxes. You should take care of it, at least to about 70% to make people think they are doing well.
Maintaining a constant influx of people is not very difficult. The basic element of their satisfaction is work, so create new plots according to your plan of specialization or look at resources and try to fill up deficiencies. When new jobs are created, entrepreneurs will let you know which employees they need - then build several new housing plots.
However, remembering about everything that people need is impossible so after a while people demand the objects you've forgotten about - sometimes it is a shop, sometimes a swimming pool or a playground for entertainment. Executives and Elites provoke most problems as their level of compliance is much higher than the one of lower classes.
Remember about streets' passability - after reaching 50,000 of inhabitants public transportation becomes available. Buses can greatly decrease traffic problems if they are carried on in the right way. For the convenience of the residents, try to build as little as possible zones of unrestricted shape, choose rather rectangular ones as in rectangular plots bus stop reaching areas are much larger. The working class will be happy to benefit from such a means of transport.