So, I got all my parts for my PC and about to put it together tonight but haven't secured a copy of Windows 10 and was thinking about just going with Linux.  I had a coworker said he could get legit Windows 10 Pro keys for pretty cheap but not sure about that and I haven't been able to get a hold of him.  So I was just wondering what my best option would be for someone that is looking to go Linux route but has only used DOS and Windows entire life.

I bought my Windows 10 Pro key online for about £14, works perfectly.

Anyway, for a new linux user I'd recommend Ubuntu.

edit: http://www.mrhightech.net/operating-systems/3554-windows-10-professional-0885370903911.html

^ That's the site I got mine from, £12 at the moment. Though if I remember right they don't send the key instantly, think I had to wait several hours at least.

Just not sure what are the legit sites.  Don't want to get ripped off and have to deal with reporting it to card company.

I would recommend Linux Mint. It's based off of Ubuntu, but has a better release schedule because it uses the Long Term Support (LTS) releases as the base. The functionality is pretty old-school Windows, which I think is great.

Linux isn't for noobs X: Ubuntu would be the most popular though so by virtue of that it probably also has the best and most documented support.

Do most steam games work for Ubuntu?  That is basically all I plan on doing with this computer is play video games on it.

Ubuntu if you want a Windows that can't play games, Debian if you want to start Linux.
Be sure to try it out on a virtual machine first, and test multiple UIs on it.

So my best bet is to just go with Windows if all I want to do is play Steam games.

I'd go with Ubuntu. Most support, largest community, most problems will be answered for Ubuntu.
In my experience, it has the least problems playing games on Steam on Linux (next to SteamOS).

Mint. Is easy to use and the interface is similar to Windows XP. It also has an useful Software Manager that automatically downloads and installs programs. Think of Windows Store but with free software. So the Mint experience can be very headache free, with little to no need of using the Terminal (command prompt thing) to install software.