First off, don't sweat it--most of what you learn in one language or implementation will transfer to others, so you're okay on that score. Scheme is generally regarded as being optimal for learning if you're new to programming, but it's not set in stone. If you're running some Linux distro, and you've got Guile on your machine already, that's great. If you're running something based on Debian, like Ubuntu, just type "aptitude install mzcheme" and you're done. For learning purposes, Scheme is fine, and the free online book How To Design Programs (htdp.org) will get you going.
If you're set on learning Common Lisp, Land of Lisp just became available (landoflisp.com), as did the newest release of free LispWorks Personal Edition (
http://www.lispworks.com/downloads/index.html). Or you can download SBCL from sbcl.org if you're running Linux, they're all fine.
The world is fragmented because neither Common Lisp nor Scheme was the work of a single person, so there's no "benevolent dictator" who has the authority to designate the reference version.
But don't let any of this discourage you. Read this page:
http://wiki.alu.org/RtL_Highlight_Film
and hopefully it will inspire you to proceed and enjoy.