Hey there Logan_14. Happy new year!!!
Basically you can turn most external drives into internal ones just by removing their external enclosure, but I strongly recommend that you don't do that with the My Passport HDD. Some of our portable drives have proprietary connectors, which means that you won't be able to use them internally via a standard SATA connection or with another enclosure. Furthermore this voids the warranty of the drive and on top of that, if the HDD has hardware encryption and you have data on it, you won't be able to get it without the drive's original enclosure. So basically it's up to you if you want to take those risks, but I'd advise against it.
On the other hand, if you need help with choosing an internal HDD for your laptop, I'd be glad to assist you.
About the Windows installation - if your Windows is OEM (if it came pre-installed on your laptop's hard drive), you should download the exact same version from Microsoft's website and create a bootable media (you can use a DVD or a USB flash drive) to install it. You should have a sticker on the bottom of your laptop with the product key for your version of Windows. If you don't have one, I'd recommend that you get in touch with the laptop manufacturer's customer support and ask for assistance.
If you have a retail copy of Windows, you just install it as usual.
Please let me know if you have any questions whatsoever.
Boogieman_WD