The best instrument for learning about the combination of melody and harmony is the piano or any type of keyboard. This is because the notes appear as black and white keys and this makes it easy to understand what is happening as you play. As you have already learned, there are only seven letter names used in music - A B C D E F and G – the musical alphabet.
On the keyboard, these notes are the names of the white keys. After you reach G, the pattern goes back to A again. If you go higher or lower, you get higher and lower repeats of these notes.
An example of this is as shown on the treble staff and then shown on the keyboard below the staff. Sharp and flat notes are the black keys on the keyboard. If you have a friend who plays the keyboard, ask them to show you a bit about what they are doing.