I'm wanting to teach my horse some Dressage but I have no idea where to start. What should I start with and how would I do it?

Take some lessons with an actual dressage trainer so they can help you and your horse specifically
Start with building his top line, and get him working over his back. Then try to encourage his poll down and always riding him supple. Then get him bending
You could download a beginners dressage test online, and work on it a few pieces at a time. Basic Dressage mostly consists of circles and straight lines, so nothing crazy.

The main things you will want to work on are engaging the horse's hind end, and keeping your horse straight, especially as you're doing your circles. Almost think of them as hexagons, because you don't want your horse to over bend, because this means that your horse isn't balanced and isn't using himself correctly.

There are a lot of great resources and articles, but lower level dressage is just making sure that your horse has a solid foundation of the basics, and is responsive to what you're asking of them.

I would worry less about being on the bit and be more concerned about straightness and forwardness. If you don't have those two things first, then being on the bit isn't going to be too helpful. There are lots of videos on youtube you can watch, so just do a quick google search, find some resources, and you'll be fine!
Learn about on the bit
At least that's where I would start
He needs to have baisic training to start. Assuming you have this, you want to strengthen his hind end and back, and then a good place to start is usually lateral work and extending.
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