My biggest suggestion to all those new to driving: Start with an experienced horse and driver with you! It's like learning any other new equine sport, it's easier to learn the correct cues, commands, and other tidbits with someone there to tell you what/how to do it, and a horse that is going to give you the correct reaction when you finally do get the command right.
I started out with horse drawn carriages and drove with an experienced driver on the box with me for the first 30 or so drives. I've been driving for 7 years now, and am just now starting my first horse by myself.
Starting driving, be it new driver or new horse, is not for one person, you really must have someone else. Make friends with someone locally who drives and ask if you can go out with them and watch them drive, then drive their horse. And ask if that person can go out with you and work with your horse the first few times to make sure the horse is as well trained as you've been lead to believe. If your area hosts any CDEs (Combined Driving Events), go out there and watch, meet people, etc. CDEs take 2 people, that person hanging off the back is vital, and many drivers would gladly teach you to drive in exchange for you navigating for them.