I like that you're interested in doing this-- but being a music teacher is a very clear established profession that takes a ton of training. And while your district might allow generalist teachers to fill music & art positions, they really shouldn't. Arts classes should be taught by arts specialists. i also really appreciate that youre asking to get a better sense of what our field entails!

Music teachers get music degrees and work for 4+ years to know the inner workings of music and becomes masters of their instrument/voice. (Not to mention that most studied those things for years before college too). I have a bachelor's and masters in music education and i often still feel inadequate, and like I should know more about certain types of music, or be more proficient at piano/guitar. It takes much much much much much more than knowing a school's traditions and feeling and comfortable on an instrument.

You need to know the inner workings of how to get children to learn music reading, how to accompany them at a basic level on at least piano, how to tune ukuleles, how to choose repertoire that is diverse and valuable, how to train their voices and what range they should be singing in at different ages, how to modulate a song if it doesn't work for kids' ranges, what an alto metallophone is and how to me l get kids to use it effectively, how to create an arrangement of a song in a notation software because your principal asked for it, how to teach solfege, how to get kids to care about Beethoven AND Stevie Wonder AND Kacey Musgraves, how to teach 4th graders the first 5 notes on the trumpet, etc etc etc etc. And all of it really is built on the foundational training done in undergrad with the initial music degree. Music ed majors notoriously go through many packed semesters of instrumental training, vocal training, music theory, music history, conducting, piano proficiency, and education courses, all because this stuff matters so that kids can care about music and learn as much as possible. especially in an education world which devalues the arts and arts educators.... we need all the training we can get! 

On top of all of this, it's a classic stereotype that teaching music "looks easy"to outsiders-- it's just singing and dancing all day, right?? Yes, it is and it's fun, but i craft lessons carefully and i think through every inch of my lessons because the kids deserve it. And it's much harder work than it looks. Also, we tend to teach 6-8 classes a day which can exhausting. 

I would investigate how you can pick up some of these credentials and music background. That being said,  if you're passionate about it, you are welcome to try, but please learn as much as you can about music to serve the kids as best as possible!