First, you need to figure out the musical key of your song. If you discover your song is in C major, for example, then you will be able to play the following notes over it: C D E F G A or B.
These types of songs are called "remixes". You will have to recreate the song, but you can search up the melodies/harmonies/etc. of the song to copy them to your song.
Note where middle C is on both clefs (for viola it's the center of the staff, piano treble clef it's the first ledger line below the bottom of the staff....
