The best instrument for learning about both 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 very easy to understand what is happening on the keyboard as you play. There are only seven letter names used in music - A B C D E F and G. These notes are called 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 shown on the staff using a treble clef, and on the keyboard underneath. If you have a friend who plays the keyboard, ask them to show you a bit about what they are doing.