I think it's great that you're wanting to give your horse something new to do :) And western is a BIG change!
1) When you think of a good western pleasure horse, you think of something very slow, right? Those good ones are slow from their legs. If you watch those legs, they move slow and have a solid 1-2-3 rhythm. That's great breeding! You'll see others that aren't moving any faster, but if you look at the legs, they seem to be going 90 mph. Those are the horses that are trained to go slow. That's a whole heck of a lot different! So first, take a good look at your horse and see what his legs are doing. That will help decide how you'll train him.
2) Whoa is your best friend. We want our horses to work entirely on a loose rein, focusing more on leg, seat, and occasionally voice aids. My #1 rule is Whoa means Whoa, and don't say it unless you mean it! Our slow down cues comes from my seat, guiding from neck reining and my legs, and stopping is seat and Whoa. He has to learn how to carry himself in a balanced frame with a relaxed neck carriage, without constant bit contact.
3) Neck reining is just basic repitition. Start riding with a slack rein, keeping your hands about shoulder-width apart. When you ask him to turn, keep your hands that same distance apart and use your direct rein a little bit which should also mean you're laying the off rein against his neck. Combine that with your leg cue, and pretty soon you'll be neck reining like a pro! (but as a caveat, most of us actually do 95% of our guiding with our legs)
4) Western pleasure and trail is kind of a different mind set. I like a slow, patient horse. We spend a lot of time walking, jogging, and standing. I stand smack in the middle of a trail obstacle to teach my horse not to rush. Think about what frame of mind your horse is in (is he energetic? is he lazy? etc.) and where you'll want him to be.
That should at least give you a basic idea of where to start :)
