There's a great book called "How to be your own Veterinarian, Sometimes." My aunt got me that book 15 years ago and it was really a godsend early on. It shows you how to clean and dress wounds, take a horse's vitals, what supplies you should have on hand, etc. It's a great reference, even for things you will call your vet for, so you know what to expect.
I keep a first aid "tub" filled with the following:
- Betadine scrub
- epsom salts
- hydrogen peroxide
- Eclipse wound wash & ointment
- pillows & standing wraps
- duct tape
- a soaking boot
- 2-3 rolls of vet wrap (or similar product)
- big box of 4x4 bandages
- thermometer w/attached string and clip
- stethescope (sp?)
- powdered bute
- bottle of Banamine (in fridge)
- bottle of Ace (in fridge)
- 6cc and 12cc syringes with 18x1.5 and 20x1 needles
Definitely get a vet and farrier set up. Make sure the vet will do emergency calls.
Other stuff you need you should already have an idea about, halter, lead, brushes, lunge line, lunge whip, saddle, bridle, pad, girth, bits, treats, shampoo/detangler, fly spray, etc. I like to have a spare halter, lead, pad, etc.