iPhone users can search iTunes and Android phone users can search the Google Play Store, refining their searches to free apps, and then further refining according to the type of app desired. Upon choosing an app to download, the user taps the Download button on the phone screen.
There are other sources of apps for Android users, such as the Amazon App Store. Both the Google Play Store and Amazon App Store offer "app of the week" deals, in which paid apps are temporarily available for free. iPhone restricts its devices to apps available on iTunes, but there are many deals that allow user downloads at no cost.