Do you have A) a horse domicile (a place for it to crash), B) a big, fenced in area for it to roam around and graze, C) a good supplier of quality hay and grains, and D) A big shovel and a strong back?
If you have all of those, you are ready for a horse. You can pick up anything else once you have it.
That, I actually can't attest to as I never boarded my horses. I'd imagine that you'd look for a place with a good reputation, and has a nice facility that they are willing to let you see all of.
Katie, I realize this post is about a month old, but I actually was born and raised on a horse farm, grew up riding and training horses, and still have them. If you have any questions feel free to message me, I have dealt with my fair share of minis and one of my good friends actually breeds them.