When you want to use an app on your phone, you usually have to find and select it from a menu. This can be done in a number of ways, depending on your phone's operating system.
If you're using an iPhone, you'll find your apps in the home screen or the App Store. To open the App Store, tap the icon that looks like a blue square with a white A inside of it. From there, you can browse through all of the available apps or search for a specific one. Once you've found the app you want to use, tap the Get button to download and install it.
If you're using an Android phone, your apps will be stored in either the home screen or the Google Play Store. To open the Google Play Store, tap the icon that looks like a multicolored triangle. From there, you can browse through all of the available apps or search for a specific one. Once you've found the app you want to use, tap on its listing and then press the Install button.