For starting out, get whatever is the official standard model, which currently is the Duemilanove. By the time you finish developing your project, you'll have learned about why you need the features offered or eliminated by the different variants, and which one to use for putting it into permanent use. Usually you can put a cheaper variant into production, because some of the more useful development features (USB perhaps) may not be needed.
For example, I started out with the Diecemila, and still use it for my main Arduino when I'm working on a project. One of my projects is being permanently installed in a RBBB, while another is going on a MaxSerial Freeduino.
But I'd still start out with a standard model because you'll get the best help from others here. Kind of like learning to walk before you try to run. 
I also started out with the tutorials here:
http://www.ladyada.net/learn/arduino/
In fact, once I found those tutes, I saw how easy it was to get started on Arduino that I bought one of those starter kits.