UL classes, try backpackinglight.com.

REI has classes and trips, but not UL. If you wanted to try Yosemite there are some guide type places available.

But classes are expensive. First time out, pick something that is a few miles in, and do a quick overnight gear test. How the pack carries, how you use everything you got - get up in the morning and go home, and review the gear list to remove stuff you didn't use or didn't work quite the same way as expected.

You don't have to go far or fast or anything to get into it. If you don't want to go alone, there's groups - Adventure Club comes to mind - huge group of Southern Californians who hike and climb and all sorts of outdoor activities. Go with the Sierra Club. Go with a meetup hiking group. Heck, PM me and come out with my group, I schedule overnight trips - doing a whole 8 miles with a bunch of novice backpackers this weekend. I may not teach, but you don't need a class to backpack. Just opportunities to do short and easy before you go do epic and multi-day.