Buy and read
Pro Drupal Development by VanDyk and Westgate. The theming chapter alone is worth the price of the book.
The book is very good, but it doesn't really cover CCK and Views, which are two very important modules that form the center of a lot of Drupal sites. Make sure to learn about those on your own.
Visit a user group, if you've got one nearby, and ask questions.
The folks at Lullabot offer tutorials on Drupal site setup,
theming, and advanced programming. They're a fun crew and I suspect their classes are pretty good, if you have the tuition and travel budget for them. Their podcast is fun also, and much less expensive. It's not tightly edited -- the individual episodes are kind of long and rambling, and you might want to listen to them with a beer in one hand, to get into the proper spirit.
I keep meaning to watch the screencasts at http://drupaldojo.com/, but somehow I never get around to it.
The non-negotiable hard part of Drupal is the theming. If you're not a programmer, but you have strong power-user point-and-click skills, there's some chance that you'll be able to assemble an ugly, jumbled version of your site. This, I think, is one of the reasons why Drupal sells so well. But then you'll want to change the order of the fields on the Comment Preview page... and you will have to spend two days or more learning the Form API and the theming system. Be sure to budget the time or hire someone who can help.