I don't post most of my photography on here, but are you intending on taking conformation shots like you see of top stallions or more at liberty shots?
For conformation shots, try to make the horse as neat as possible. It really shows in this type of photo and make sure the lighting is good. Always try to have the sun behind you, unless you're aiming to have the light on a specific part of the horse. Try and get the horse's ears forward and the front feet square. In some breeds, like QH's its customary to have one hind leg forward to show a good hip. Don't get your shadow in the photo.
For at liberty shots, (like natural running around the paddock ones), the best tip I can be is to be an observer. Sit in the paddock (or stand :p) and just watch your horse. Have the camera ready, or just be constantly taking photos. Once again, make sure the sun is behind you, because generally this provides the best lighting. Once again, don't get your shadow in the photo, unless you actually want it in there. Sometimes the best shots just happen, so just take your time and you'll pick up what works for you, your horse and your camera.
Good luck :)