The list of changes in Rails 3.0 is fairly approachable if you understand the basics of Rails. I'd recommend taking a look for yourself - http://guides.rails.info/3_0_release_notes.html

As far as whether 2.3 or 3.0 makes the most sense at this point, it depends. Personally, I have been working on a brand new project that I started with 3.0 beta 1 and haven't ran into any major headaches. That said, since you're new there is a lot more information out there regarding 2.3 (be careful, some is much older).

To give you some better advice, why are you getting into Rails? Are you trying to get a project started/out the door/etc, or simply looking to learn it because it looks interesting/fun?