If you've got a love affair with the mountains, Alpine climbing is what you need to feed the desire....
Climbing Mt Baker with one of the guided groups isn't a bad way to introduce yourself to glacier travel. If it's one of the 2 day trips the guide services offer, it's kind of a quick up the mountain and down safely kind deal. Still fun, but not focused on teaching/learning, it's up and down ASAP with minimal instruction. Keep in mind that August in the Cascades will see the snow bridges melting out, exposing some impressive crevasses. This can make it tougher (longer) to get up the mountain, particularly in lean snow years. Crossing a sketchy snowbridge will get your attention... You might consider mid June - early July for better snow conditions.. Keep in mind that most routes (if not all) on Mt Baker are glacier, snow and ice climbs. You're not going to find rock climbing, it's almost all snow and ice. You'll need to understand glacier travel, self arrest, self belay, crevasse rescue, roped travel, foot placement, etc. Almost all protection will be in the snow or ice using pickets, flukes, screws, ice axe, etc. Entirely different techniques from what you'll experience in Joshua, nevertheless all important stuff in learning how to Alpine climp. One of the coolest thing about Mt Baker is that you're in the middle of the North Cascades, anywhere you look you'll see more peaks that call you to climb them..
You might also consider a trip to the Tetons. Early summer will provide some outstanding mixed snow, ice and rock climbs. The Middle Teton has a very cool intermediate "mixed" Alpine route. This maybe a little advanced for a begginer, but probably good if you have a Mountain Guide there to teach and make sure you safe. You'll learn a bunch from the guides, it's there job and most of them really love to teach a friendly, capable, and willing student. As tempting as it is, I'd stay away from the Grand Teton for your first climb in the Tetons. Pickup the "Climbers Guide to the Teton Range" by Leigh Ortenburger and Reny Jackson. You'll dig it..
Freedom of the Hills is a must read. Like the guy above said, read it and read it again. Moreover, go get a rope, some webbing, cord and bieners. Practice what you've read and learn it. For the Pacific NW I'd also read Glacier Travel and Crevasse Rescue by Andy Selters. It's more focused on glacier techniques.
Whatever you decide to do you'll have fun.