Yeah, but first. In a key, there is only one of each letter in a scale. So instead of "C D D# F G G# A# C". It's this "C D Eb F G Ab Bb C". I know there the same note but writting them this way with help you recognize the closely related keys.

Since the melody is dark. You may want to go with the C harm. minor scale to get your chords instead of C minor.

Any melody can be harmonized with just the chords I IV and V. As a beginner I would suggest trying to work around these chords first, and limit the use of the other chords. Subbing the V chord with the viio (B D F) chord will add dark tonality. Like i - iv - vii (instead of V) - i - iv - V.

When you add another third to a triad you form a 7th chord. In C harm. minor, you get C Eb G B. A Cm/maj7 chord. I think you'll find this a nice harmony for a dark melody since this chord has a scary sound. Used alot in horror music.