When you're on your dad's computer and go to that link, there won't be any option to choose which version of Windows you get. You'll just download the media creation tool, double-click to launch it, and follow the on-screen instructions, and I believe you'll wind up with 1809, which is the build from October. (If you get 1803, from last April, that's fine too, since it's only one update away from 1809.) The whole process is quick and painless, although it might take a bit of time to download the ISO file and burn it.
Then you can plug the USB into your own computer, launch Bootcamp Assistant, and follow those instructions. This is also very simple, if time-consuming. The one thing you should think about now is how much space you want to devote to Windows; you won't be able to change it afterwards unless you wipe the partition entirely. (This is also easy—just a couple of clicks—but you'd lose all the data on the Windows side.) For Sims 3 with all or most expansion packs, plus saves, custom content, etc., you'd want at least 256 GB, because Windows needs a lot of free storage to maneuver. Even then, you might need to store your extra save files on an external hard drive. These are pretty inexpensive; I bought a 4 TB drive for $110 last year. If you're going to do schoolwork on the Windows side, you might want to increase the partition size to 300 GB. But it depends on how much you want for the mac side too, so take some time to think about it.
This is what my mom bought from Amazon: https://www.amazon.com/Windows-Home-English-Flash-Drive/dp/B07Q3X32QT
The "Fulfilled by Amazon" is what you want to see—it means Amazon will take responsibility if the product doesn't work for some reason.
It came in two days with Prime, and it included the USB stick I didn't use, plus a code printed on a card. Searching Amazon can be annoying, especially for a term like "Windows 10" that's included in so many other products, so I tend to use google and specify Amazon as the site. For example, I used "windows 10 home USB site:amazon.com" to find the above product.
A word of warning: It's not a good idea to buy an OEM version. Microsoft doesn't support its indvidual use; it's for manufacturers that ship their computers with WIndows installed. Plus, the code can't be transferred, so I'm not sure you could still use it if you wiped your partition. I mean, you should be able to reuse it, but there's always a chance it wouldn't work, and then Microsoft wouldn't help. So it's better to spend a couple more dollars and get the full version.
Oh, and when I said Amazon Direct, I meant not Marketplace, which is Amazon's attempt to create a slightly more civilized version of ebay. You could probably find a Windows 10 code on Marketplace for $10 or $20, unless Amazon has cleaned that up again. But the code might well have been stolen, and it's always possible that Microsoft could figure that out and cancel your access later, much like EA has done with stolen TS3 codes over the last couple of years.
