If this is new to you here’s a diagram to get you started. The notes that sit on the staff lines are (from bottom to top) E G B D F. A common nemonic for this is Every Good Boy Deserves Food, or Every Good Boy Does Fine. The notes that sit in between the staff lines spell (again from bottom to top) F A C E.
(Note: The app defaults to the key of C major. I recommend you leave it there to start with. For the purposes of learning harmonica we always think in the key of C.)
For a blues harmonica player sight reading isn’t an essential skill to develop but it’s certainly useful. I’d encourage everyone to try. The app is fun to use and you’ll be surprised how quickly you improve. I’m sure something similar is available for Android devices. Maybe somebody can suggest one?