For those of us who are a little more Type A, there are several kinds of prep work you can do first. I always like to read up on whatever it is I'm learning, so my first advice is this:
In fact, this is exactly what I did last week; I went to the baking section (shelves and shelves of gorgeously tempting cookbooks) and checked out everything they had on cake decorating for beginners. Well, everything with good pictures.
I wound up with two books that teach basic technique for more traditional cakes, and two books about cupcakes that I just have to show you: Hello, Cupcake! and What's New, Cupcake?, both by Karen Tack & Alan Richardson. They're full of exactly the sorts of ideas that get you noticed on Cake Theater...and oh, dear, they have tutorial videos.
I might need to get back to you in an hour or so...
Hey, that brings me to my next suggestion:
This one is awesomely retro, and it's a series! It's pretty basic, so it's good for if you're just getting started. There are many more helpful videos on YouTube, so if you learn best from videos, take advantage of this! Try searching "cake decorating" or "creative cakes"...see? Hours of video goodness.
...because nothing keeps you going like reading how other people do their thang. I just found this beautiful blog, which I promptly bookmarked, and a list of 50 cake decorating blogs which I'll also be poking through sometime. My first love in cakery-bloggery is still I Am Baker because she decorates the insides of her cakes...!!!
If exploiting the Internet and the lovely free library isn't doing it for you, you can always learn from a person. Craft stores sometimes offer cake decorating classes (try Hobby Lobby or Joann), or if you have a friend who's a good cake decorator, you can ask permission to hover next time they make a cake. You could even offer to trade favors for formal lessons.
After all that...reading, watching, chatting, plotting...jump in and get messy! All that prep work by itself won't make you any better; it's just information that will help you learn more efficiently once you're actually practicing.
I'm going to say it again, and not for the last time: the best way to learn to do something is by doing it. Make a cake. Try your new skills. Keep a journal if you like, of what you did and what you learned. Take a picture of every cake so you can watch your progress. When you're ready, go ahead and send us the pictures you're proud of...we want to see them!