Kinect Fitness for Xbox One
This is not an app for your smartphone, but it is a great way to get in shape using technology that you already have in your home. If you own a Xbox One with Kinect, you have what you need to start training with virtual professionals.
There are free and paid options to help you get in shape, and the Kinect can tell you if you are doing the exercise right, so that you don't need to worry about wasting time with bad form. There are many trainers to choose from and you can track your progress with the Kinect Fitness app.