First off i will give a VERY brief explanation of what an Operating System (or 'O.S.') is.
The OS is the 'main program' on your computer, like the conductor in an orchestra signaling to all the musicians when it's time to play their parts, the OS signals the different programs when its their turn to do what they have been designed to do (if they are allowed to do it).
Some examples of an OS are Windows XP, Mac OSX Mavericks, Windows 7 & Debian Wheezy.
Yes, there is an alternative to MS Windows & it's FREE (& yes I'm ignoring MAC - with good reason)- free operating system, free office software, free anti-virus, firewall & more. Thats free as in beer & free as in freedom. The main thing about Linux is that its open source (this means that all of the code that makes up the programs is available for any person to read/scrutinise - & modify for their own use if necessary). While Linux systems used to be very fiddly, things have really changed in the last few years, especially the Debian based Linux operating systems like Mint, SimplyMepis & Ubuntu both easy & a pleasure to use.
The first decision is what kind of PC you're going to run it on. I'd suggest using an old clunker to begin with - so that if (ok, when) you do screw up, it really doesn't
matter). This method also has the major advantage of allowing you to look things up on line on your other PC until you have a decent idea of what you're doing.
If you do go with an old machine you may as well just 'give it a go' ... googling any questions the installer asks you as needed. If you're going to install it to your main PC I'd really advise getting someone to walk you through it (or do it for you) its WAY too easy to screw up & wipe your main drive by mistake (not good:)) the newer installers have clever ways to reorganise your Windows partition to fit Linux on the free space - it'd probably work but i for 1 wouldn't rely on it - neither should you.