redhat, mandrake and suse are what most consider noob friendly if you plan on using a GUI. If you plan on going CLI only, then there is really not much difference from one distro to another. redhat has a lot of command line apps that make admining a server box easier that are installed by default (netconfig, ntsysv, etc) though there are equivalent binaries for other distros.
Debian is a good choice if you really want to learn a solid linux distro. their package management is second to none, as is the quality of the packages themselves.
edit: slackware is probably not what you want to start with...at least in my opinion