Well there's the 2 well known Linux based operating systems:
~Ubuntu 7.10 or beta release 8.04
~SUSE Linux 10.3
These 2 have free download link to get the OS but to download it, it is recommended to have the user eatheir having DSL or Cable modem not dial up.
For my recommendation get SUSE.
So here's an step by step on how to download Ubuntu 7.10:
1st: Choose your type in this case get Desktop Edition 7.10
2nd: If you have an brand name type computer such as Dell/HP choose the first button. If it is an custom built and is 64-bit choose the 2nd bubble, and basically the Mac/Apple types are the 3rd bubble
3rd: Choose your location
4th: Press download ~it may take an while because since it's an operating system.
Here's an step by step on how to download SUSE Linux 10.3:
1st: Choose between KDE or Gnome personally I'd like you to try out KDE
2nd: Choose between 32 bit if you don't know what type your PC is, ^$ bit to take advantage with this one if you do know if you have 64 bit, and Power PC if you have Mac
3rd: DVD or CD might want to choose CD
4th: Choose your download style if you do get CD format choose the Live CD section.
Just message me any time When you still need help.