In the addition of the obvious "know what you are teaching" (in your case dance) you need skills to teach. Not everybody is able to explain what they know to others. It requires some psychology based on the age of the student to be able to transmit what you know and them to assimilate the information.
You also need social skills.
- good communication skills
- good psychology skills. Those include how to keep people focus so they listen to you.
- good skills in the topic you want to teach
This is a great question since being a dancer can be quite different than teaching someone else how to dance like you.
I would say that the first thing that you need to consider as you think about being a dance teacher - what do you want to teach? I took a dance class for several years - Flamenco dancing. My teacher had studied Flamenco for several decades and she was versed in various styles of Flamenco. So if you want to specialize in a type of dance - such as Flamenco or Ballet - you are going to want to be well versed in the different styles.
The second thing to consider is - do you want to teach more than one type of dancing? My Flamenco teacher actually taught a variety of classes along with Flamenco - Ballet, Tap, Jazz. In that case, she needed to know about the different styles within each category. Ballet can have various levels with the most advanced that I saw being Point. The popularity of some styles may vary by where you live, so research was is happening in other dance studios, what classes are popular.
You should also consider who you want to teach. Teaching toddlers can be different than teaching the elderly. You need to know what the different body types are capable of. Some dances, like Flamenco, have elements that are traditionally male or female, so you would need to learn the differences.
Of course, there are some general elements for being a teacher to consider. Not everyone learns the same, so you will need to be able to teach people differently. Some people benefit from good verbal directions. Some people just need to watch you to do it. Some people won't learn it the first time, or even the first ten times you explain it. Do not be afraid to let other students help you teach someone a new move.
My Flamenco teacher was great. However, she assumed that everyone in her class had taken some form of dance before. I had not. When we did warm up, she was always using Ballet terminology, which I never understood even after five years. I just did what other people were doing. When she was talking about a rhythm in Flamenco, she would explain some of the more complex elements using analogies of tap dancing. Again, I never took tap so I just had to watch and learn. While that language was short cuts for others, it always made me feel like I was an idiot. I wasn't. My teacher was using shortcuts that she was comfortable with rather than teaching me on my level. I figured it out eventually, but it shook my confidence the entire time.