When creating a geospatial application, the data you use will be just as important as the code you write. High-quality geospatial data, and in particular base maps and imagery, will be the cornerstone of your application. If your maps don't look good, then your application will be treated as the work of an amateur, no matter how well you write the rest of your program.
Traditionally, geospatial data has been treated as a valuable and scarce resource, being sold commercially for many thousands of dollars and with strict licensing constraints. Fortunately, with the trend towards "democratizing" geospatial tools, geospatial data is now increasingly becoming available for free and with little or no restriction on its use. There are still situations where you may have to pay for data, for example, to guarantee the quality of the data or if you need something that isn't available elsewhere. In general, however, you can simply download the data you need for...