The City Hall is mainly a economic building and it's purpose is to collect taxes. it takes up 3x4 tiles and is a government building. It is unlocked at experience four. you should have about 9 residential buildings by now, so each building will contribute to the taxes. The higher the level of building, the more tax it will give you. So a small residences in a neighborhood like this: (on the top)
Would not give you much. On the other hand, a Skyscraper such as Platinum tower would give you more Simoleons: ( on the bottom)
Lots of Simoleons there. However, the happiness of your city also effects your tax income. Just like real people, Sims are happier when they get good service and specialization. The more special services you give a certain area, the buildings in that area are usually 100% happy and therefore are more likely to be a premium or luxurious building. Collecting taxes is easy. When your city hall has Simoleons to offer, a large spinning Simoleon appears above your city hall. tap on that Simoleon and you will get your income of Simoleons. Taxes are just one of the many ways to earn Simoleons.