Welcome to LQ!!
First, we'll need to know if you have an Intel Mac, or a Power PC. There are two basic choices when picking a package to download: CPU archtitecture and 32-bit vs 64-bit versions. 32-bit will run on both 32- and 64-bit machines, and I recommend sitcking with 32-bit at the beginning.
Gnome is a desktop environment, not the name of a Linux version (distro). To see the most popular Linux versions, go to http://distrowatch.com
Among other things, you can see which versions use Gnome by default.
Once you pick a distro, download a single .iso file for one CD. Most mainstream distros have this option. Once you have the .iso file, use the "burn from iso", "burn image",..or something like that....in your CD burning SW.
When you have the CD, reboot the computer, make sure the BIOS is set to boot from CD, and boot up from the CD. From there, it is usually self-explanatory.