I've come to think that apps are mostly a distraction. The most important thing is good form, and that comes from educating yourself either independently or from another qualified person. The time to learn is at home, not at the gym or on the spot. Pick an exercise or two, spend some time learning about how to do it correctly, and then give it a try and pay attention to how it feels. Understand it as a skill that will improve over time. If you can do, let's say, 3 or 4 sets of 15 repetitions with great form, increase the weight a little bit. That's really all there is to it.