Advice from the Federal Trade Commission website on downloading and using apps:
You need a smart phone or another mobile device with internet access. Not all apps work on all mobile devices. Once you buy a device, you’re committed to using the operating system and the type of apps that go with it. The Android, Apple, Microsoft and BlackBerry mobile operating systems have app stores online where you can look for, download, and install apps. Some online retailers also offer app stores. You’ll have to use an app store that works with your device’s operating system. To set up an account, you may have to provide a credit card number, especially if you’re going to download an app that isn’t free.