We trail ride a lot, I always leave a simple rope halter on my horse under the headstall that way when we take a break I can use the rope I have on my saddle to take the bit out of my horses mouth and just tie her if I want. I also carry everything I need on me in a large pack around my waist. That way if for some reason (hasn't happened yet thank God) we get seperated I have my water, cell phone, grainola bars, chap stick, GPS, camera. I have another pack I keep on the horse for things I wouldn't really need if we got seperated horsemans knife (it has hoof pick in it plus other assorted tools) horse cookies, some fly wipes, plus more water. I wear a palm straw hat that I can put water in if need be for my horse. I have a cheapy watch attached to my saddle too. I guess that's it, geesh that seems like a lot of stuff! Just a suggestion don't buy the saddle before you buy the horse :)
Just wanted to add you will need all the grooming stuff too brushes curries etc. Hey nobody said horses were easy or cheap
I suggest you head to your local tack store and don't be afraid to ask questions, its the only way to learn and that way you will get some idea what is needed.
...when I ride western I have a saddle bag with me, in it I carry a flash light, some times rides can go longer, bug spray, a good pocket knife, rope the rolled lundry line type( you never know when you might need it), snack..you never know when you'll get hungry, halter and lead rope.....i hate cell phones (it's my time between my horse and not the world and I) , unless you only plan to ride in the rain, the oilskin coat is not a have to have itiem ( I have one.....great dust collecter in my closset) and last a hoof pick 
