Welcome to LQ.
Linux isn't difficult - you just have to get started. We all started knowing nothing
Back up your personal files in case of mistakes.
Choose your distribution - I'd recommend kubuntu 6.06.1LTS Get it here:
Burn it to a CD as an iso file (Important!)
Defrag your windows disk(s).
Boot from the CD (you may need to set this in your BIOS), it'll run from the CD (slowly), then click Install to install it.
Follow the default recommendations. It'll automatically install grub as your bootloader, which will give you a choice of linux or windows at boot time.
If you encounter problems, search this board (see the SEARCH button, above?), or provide details of what is wrong and ask again here.