Hi Oliver, welcome to the group!
The first place to go is
http://beagleboard.org, the portal for all things BeagleBoard. The System Reference Manual is there, which is a great document both to start with and to refer to.
In my (not humble) opinion, one good place to start is with an article I wrote last year on getting started (
http://www.ibm.com/developerworks/linux/library/l-beagle-board/index.html). It discusses the kinds of cables you will need to make the rev. C board work. You need fewer cables with the xM, as it has an onboard USB hub and ethernet port. In general, it costs about $200 to get started with either board, assuming you already have a monitor and USB keyboard/mouse you can use, if you want to use those things.
Another good place to get started is the wiki at
http://elinux.org/BeagleBoard. There is a ton of information there and a lot of links to follow.
I am currently working on a book on the BeagleBoard that should be available toward the middle of next year.
Good luck,
Jeff