IMO, learning to trim is pretty simple.....if you're willing to dedicate the effort into learning the subject.
I purchased a dvd set and spent hours on the internet learning. Actually more than hours, it became an obsession. All of my free time was spent learning about the hoof for a couple of months. I really took to heart the saying, "no hoof, no horse", so I gave up tv and other pointless activities and focused my time to learn as much as I could. And of course the dvd set is something like 20 hours in length and I watched that probably four to five times, and would also refer back to it. But I don't want to suggest that anymore because I learned something a couple weeks ago that totally altered the way I approach barefoot trimming, which is hoof mapping.........
Google hoof mapping or elpo hoof mapping, and get on youtube and watch thehappyhoof videos ...... actually here's a link thehappyhoof - YouTube
she also has a yahoo group..... here it is, thehappyhoof : The Happy Hoof
. When I started out learning to barefoot trim a few years ago I was also learning from her videos, and she no longer does the type of barefoot trimming I learned back then. She now does a barefoot trim by hoof mapping and she is getting better results. If you watch all of her videos where she is doing hoof mapping you'll get a pretty good base of knowledge built. And just google the terms you don't understand.
But now that I've said all this....if you're the type of person that doesn't learn very well by self teaching then I suggest what others have said. Learn what you can on the internet, but find yourself someone that can also teach you the experience. Though the downside is that they may not be up to date on what will give the best results. But IMO, if your horses are pretty sound and don't have any serious deformoties of the hoof then it's really pretty dang simple......if you're willing to dedicate the time.
And really it's my opinion that if you have a horse you should know about the hoof and how to maintain it.
Here's one more site..... The Barefoot Horse - an Introductory Guide to Barefoot & Booting
I really was unsure if I wanted to post this site, because if you learn some of this stuff you may need to unlearn it as you learn about hoof mapping. But I think over all it will help add a stronger knowledge base.