Hi Taki. Let's say you decided to test Ubuntu. What you do is download the ISO and then burn it to a disc. Place it into your cd-rom and start your computer. If your computer is set up to boot from a disc, it will look for a boot file on your computer. If I remember right, Ubuntu will bring up a screen asking if you want to boot from the cd or proceed with a full installation. Select boot from cd and then you can test our your cd.
Now...here is my disclaminer. If you follow these instructions and your computer gets messed up, you will not hold me responsible for any damages nor will you flame me in this forum. Ok?
For more info, try this link:
http://distrowatch.com/