You can find the tuner when you open a song: it is the little tuning fork icon. The changes you make in the tuner are saved, so that for all songs the reference for pitch analysis is the same.
When you play a note, the tuner tells you which one it is and shows you the pitch with a transparent bar that goes up and down. There is also a line that shows where the bar (your pitch) should be.
If the bar stays below the line, your pitch is too low, if it is above the line, your pitch is too high. This is also indicated by the color of the line. The goal is for the bar to reach the line so that it turns green.
To achieve this, you can either tune your instrument yourself and try again, or you can drag the line until it matches the tuning of your instrument. That way our app knows what your optimal pitch is, and you are ready to go.