My main PC needs to be upgraded from Windows 7, but I've never upgraded an operating system, I've only gotten new computers or built one fresh. I'm not as knowledgeable when it comes to this.

I wasn't going to bother backing up the PC because I do not have a spare hard drive other than the other empty 500 GB HDD that's on my PC. Worst case is I lose all the files and then reinstall the games I want to play, I do not have any files I care about enough to justify buying another hard drive just to back it up.

What I need to know is if it is as easy as going to the store to buy a Windows 10 disc, inserting it, and just following the on screen instructions? Or are there a bunch of other things I should be doing to prep?

I think you shouldn't do it. Windows 7 is a great as is and depending on your computer specs windows 10 might run slow on an old windows 7. Plus I like my old little windows 7, its got some miles on it. I am pretty used to how it plays. That probably wasn't of help but I thought I would mention it.

For storage you can use things like dropbox or google drive to download everything into the cloud and onto another computer, or in this case the same computer. A bag of chips and salsa a month goes a long ways in the cloud these days.

I do enjoy my Windows 7, and I make no use of any of the special Windows 10 features on my work laptop, but the main reason I was going to upgrade was for security purposes... isn't it wise to upgrade Windows purely from a security standpoint?

It is don't listen to him

go to ebay but a windows 10 pro x64 oem key for 3$
then micro$hits website and download the iso/usb tool.
get a flash drive and flash it with iso.
then boot from flash drive and follow prompts like a normal install.

why are they so cheap on ebay vs $100+ to buy the disc and everything off amazon/walmart/microsoft etc..?

also is there a way to verify the key is good before buying it? did a quick google and found this: https://www.***com/itm/WINDOWS-10-HOME-KEY-32-64-BIT-GENUINE-LICENCE-ACTIVATION-OEM-CODE-WIN-10-HOME/324011585812?epid=127608316&hash=item4b70987114:g:hT4AAOSwUrxd843O

they just send you a key via email or whatever?

they're cheap cause they were sourced from old machines and legally they had to provide keys and in eur and other regions they're legally allowed to resell em.
so the options for them were to toss em or sell em.

The link breaks cause its ebay and i'd suggest you not try and post it again unless you want 10%
But yes they just email it within a day or two, in the mean time you can install with out a key and activate it after.

search again and filter buy buy it now and lowest price = $3.22 saves you a bit over 50cents.

Well I'll give this option a try first then versus buying a new windows 10 home from walmart or whatever for 100+

the only limitation of those oem keys is they bind to the hardware ID of the mobo so when you change pc's/mobo's you may require a new key.
But at that price point why wouldn't you use them. I've put those keys on hundreds of pc's and no ones had an issue.
Occasionally you get mailed a key that's already in use if this happens fear not just shoot them an email and they replace it.

Is there anything to look for in terms of reputable dealer? I never shopped for keys, and never bought anything on ebay, so I have to set up a paypal lol

Paypal/ebay always backs the buyer generally. so you really have no concern even if scammed.
You buy another key and open a ticket for the scammed one, they give the seller a few days to try and argue their point.
while the ticket is open it effects the seller and so their are quick to fix things, if no response or they can't prove their side they refund you instantly.
But as stated above msg them first and wait 24hrs as to not flag an otherwise honest seller.