I'm not sure about apps, but there is a great book to help you understand how aperture, ISO, and shutter all come together to create proper exposure called Understanding exposure by brian peterson( I think) I also second putting the camera on manual and shooting. When I learned all of this, film was the only option, and I would write my setting down on a piece of paper,a nd rake a shot of someone holding the paper. I did this with an entire roll of film. When I got the film back, I looked at the shots to see what changes had what effect. I also just walked around shooting,a nd writing settings down in a notebook, and studying the shots compared to the settings to see what different apertures, etc did. You hae an advantage of being in the digital age. You don't have to take notes or wait for film! You can pull images into photoshop or something like that immediately after taking them, and examine the settings, and compare two images right away!
Good luck,a nd enjoy the journey! It's a fun one!