Dear, If You want to create real time alert then GCM (Google Cloud Messaging ) is best for Alert and Notification on device.If you want to local alert like data pack off Alert then Alert dialog and Toast is the way by which you can alert the user

Android Tutorial- http://www.hub4tech.com/android-tutorial

http://stackoverflow.com/questions/12600360/how-to-make-an-alert-dialog-box-in-android

http://stackoverflow.com/questions/26097513/android-simple-alert-dialog

http://stackoverflow.com/questions/12600360/how-to-make-an-alert-dialog-box-in-android

http://www.wikihow.com/Show-Alert-Dialog-in-Android