Let us know the specs of your computer before we get too far on advising
you. Make, processor, RAM, HD size and configuration, etc.

The first step is to download an ISO file for the Ubuntu version that you
plan to run. But, you may need to reconsider depending on the age of the
machine. Ubuntu's latest versions are comparable to Windows 7 or OS/X and
not XP. There are lighter distributions including several in Ubuntu's
parent company, Canonical's, stables. Xubuntu and Lubuntu are the same
underneath and use the same repositories, but use lower resources.