Check the Ubuntu.com page
https://help.ubuntu.com/community/Installation/SystemRequirements
and the Ubuntu Forums (another good place to get info on Ubuntu)
http://ubuntuforums.org/showthread.php?t=24890

The software you DL and burn to a CD, the CD is then bootable with a copy of
Ubuntu that you can run straight from the CD on your computer. No
installation to the harddrive. That is providing you computer CAN run
Ubuntu. Note: Running from the CD WILL be much slower because CDs are much
slower than HDs.

Get Ubuntu here
http://www.ubuntu.com/getubuntu

I have ubuntu running on an old laptop with specification of Pentium II
(333MHZ), 192MB RAM, and 6GB Hard drive.
It runs ok albeit in a not very snappy fashion.

Once you get a low level spec i.e RAM of 128MB or slightly less it is worth
taking a look at xubuntu
http://www.xubuntu.org/

As this is designed with low spec hardware in mind.