Get a mouse, keyboard and operating system
You can find a keyboard and mouse set for $30 that will be good enough to get you going (really, those are a dime a dozen). And while there are a lot of gaming specific keyboards and mice out there that give you all kinds of fidelity and techno jargon, those are not a necessity for your gaming PC.
Now, in the way of operating system, Windows is the one that this specific PC build is geared toward and the OS almost every game you will want to play is optimized for. The issue here is "Which version of Windows do I get?"
I use Windows 7, but Windows 10 is Microsoft's newest offering and you can find deals all over the internet for a decently priced key. Once you have this, you will be up and running!