Software questions from new DIY'er

I first wanted to apologize if there is an article covering this, but I lost my patience trying to find one - Thank you in advance for taking the time to read this.

I'm new to building a computer, in fact I am considering embarking on my first build it yourself computer. I have spent about two months reading articles here and on other sites about what hardware I wanted to buy, and have come up with what I am ecstatic about out. One thing I hadn't thought of before is the software side of a computer!

So from someone who knows very little about computers but is an avid gamer - If I buy all the hardware, what software do I need to buy to get my system up and running? Does buying the operating system cover everything that I need? I would appreciate any advice (or a link to an article on this web site that already covers it if out there) on what software I need to buy or should consider buying to go along with my new high powered gaming pc.

Thanks