You're new to operating systems
. You will have to download a .iso file, burn that file to a cd creating a "boot disk" you can then use that boot disk to format your computer...ERASING EVERYTHING INCLUDING WINDOWS AND ALL YOUR FILES...then install ubuntu.
you could partition your hard drive and install ubuntu on a separate section of it so that you could choose to boot in either windows or ubuntu.
If you just want to try ubuntu i believe there is a fairly simple way to run it strictly off of the cd for such trial purposes.
That's what I would suggest you do. Or the partition, Linux is different from Windows in a lot of fundamental ways that may make your computer very frustrating to use if you don't understand them. Try it before you commit to it.
I looked it up for you
I strongly recommend you try this first.