First you need a CD with the installation files on it. That can be
downloaded from the ubuntu website and burned onto a CD on your own Windows
or Mac computer. If you don't have a way to do this you can order the CD
free from Ubuntu, or buy it very cheaply from several places that advertise
on www.distrowatch.com .
Your computer needs to have a CD reader and be set up in the BIOS setup to
boot (start) from the CD drive. Almost all modern computers allow this.
There are several options, but take the one that says "try without changing
your computer" to start. Within a few minutes (5-10 depending on your
computer's speed) you will have the Ubuntu desktop before you. Look it over,
try things out, play with it and enjoy it for at least a few hours. You will
find that there is a great selection of free applications available which
will allow you to do pretty much everything you might need to do, including
Word Processing (MS Office compatible), Web Browsing (Firefox) and simple
games.
Note that you don't need to install it. It takes longer to start up, and you
can't save any changes, but you can continue to use it from the live CD
forever if you wish. If you decide this is for you click on the install icon
on the desktop. You can make it the only operating system or install Ubuntu
alongside of Windows. The questions the installer asks you are pretty clear
but don't forget to write down the password you enter.
I'm by no means an expert but this should get you started...