There are many apps  out there to help you sing like a pro, including the Karaoke app, the Singer's Friend and Voice Coach.
- Voice Coach offers warm up and cool down exercises as well as exercises to to help you improve your singing technique.
- The Singer's Friend app allows you to switch between different parts, such as alto, soprano and tenor. Additionally, this amazing software also shows you the exact not you are singing and works with your to extend your vocal range.
- The Karaoke app works quite differently than the other two apps in that it allows you to record yourself and share your songs with others. In fact, this app often posts competitions where you can compete against other users.
Although these applications are wonderful, my favorite is done by Jermaine Griggs. His Vocal Mastery course  teaches you everything from breathing techniques to how to sing the most difficult riffs. I have used several of his products, and I can promise you that you will not be disappointed with any of his products.