Well, that all depends on what you want to use your computer for. Mandrake and RedHat Linux distributions make good personal workstations. Slackware and Debian are really nice if you want to really learn Linux and networking and maybe do some programming and shell scripting.
Personally I'm a devoted RedHat user. There's actually a post somewhere here on TechTalk that discusses the pros and cons of different distributions. Do a search for it ;)
If you decide to go with RedHat, there's a post here: [thread]231[/thread] which explains the RedHat Package Management stuff (RPMs) which is a good place to start. Otherwise, just choose a distro and then go to their homepage. They'll over some basic info as well as where you could download free .isos of their distributions (usually through one of many available mirrors).
Let me know if this was of any help to you, or if you have any more questions!
Also ... maybe someone else would be able to offer some advice as well?