I have yet to own a horse, but the plan is to get one this summer. I want to be prepared though. What are some common ailments that a horse can get and what can be done to protect them/heal them?

Your questions is very general and to answer it, here is the section in this forum where you could see some ailments that horses might get.
http://www.equineboard.com/equine-resource-center-5/caring-for-your-horse-11/recognizing-distress-in-horses-34/

There are still so many sickness that your horse might encounter but to make sure that your horse will be healthy, you need to feed it with healthy diet and have a clean surrounding. Make a regular visit to your veterinarian to check if your horse is healthy.