Having got used to the site and obtained an accurate income, you can now think of the specialization of the city. Satisfying the needs of residents and concerns' appetites may prove in the long run very expensive. Therefore, you can or even have to focus your efforts on exporting the surplus goods outside the city's borders.
If you have large tracts of fertile land focus on food production. Farms require neither large amounts of money nor many workers but production of food can be very lucrative.
If you have an area good for tourism, you can build a lot of tourist hotels - tourism is treated as a resource and you can export its tokens offering travel services to inhabitants of other cities. Luxury hotels require qualified personnel.
Water is usually a sought kind of resource in the desert but you can find aquifers near rivers and lakes. However, the extraction and export of water is usually not that profitable because of a high cost of maintaining the pump Station. You will need a lot of it to cover the costs.
Fuel is the most consumed resource in the city (next to electricity). If you have oil fields, for example in the desert, you can build oil wells that will extract the black gold. Extracted fuel is used in your city and the surplus can be sold.
Other resources are made basing on those already mentioned or created using appropriate buildings and people. Such resources are products of industry, office services, waste treatment etc.
For virtually any resource there are several buildings that significantly improve the performance of buildings producing the product allowing you to increase export or to fill the gap when its production is not enough for the population of the city itself.
The most common specialty is investment in the farms. They are not expensive and the costs pay off quickly. A less lucrative but still profitable business are tourist hotels because you don't have to pay extra to make them function. Fuel is necessary, many maps do not have this resource at all so it has to be imported from outside.
Water for its part is a rarely exported good. Except when you have a surplus of production for the city because an extra construction of pumps is unprofitable.