Making money in SimCity isn't exactly the point of the game, but as a capitalist city building simulation, the only way you can grow your city is if you have the capital to spare. That's where your ability to make money comes in.
There are multiple ways of generating income for your city. The easiest, and arguably most profitable way to do so is to mine for alloys using ore mines and coal mines. To start things off, you should build a coal mine as soon as possible. Be warned that you'll need a lot of power and water to maintain them and keep them supplied, however, so be prepared to spend quite a bit on your infrastructure.
If you build a smelting factory, you can get even more money from what you mine by exporting those alloys to the global market. Through this method, you can make upwards of a million simoleons each month.
Once you have an operation built up, you'll need trade depots—and a trade port later on—to export your stuff, or they'll stop working once the storage fills up. A trade port will allow you to efficiently manage your resources.
The whole operation will come at the cost of high pollution, so be sure to place your residential areas away from your industrial zones.
Your city may be operating at a loss, but it'll hardly matter when you're making most of your income through trade.