I started taking pictures using a Nikon 3200 camera. Where do I find photo software that will be downloaded to a computer that uses Windows 10?
Welcome to the Photography Life forum, @sabine384!
Any mainstream photo-editing software will be available for Windows 10. So, first try to figure out what software you want.
Nikon has their own software called NX Studio that's a good place to start, but doesn't have a lot of advanced editing options. If you want a more powerful editor, the best free option for editing and organizing RAW photos is Darktable. Meanwhile, the most professional choices are Adobe's suite (namely their photography package that includes Lightroom and Photoshop) as well as Capture One.
Again, all of these will work with Windows 10. All of them will also work with your Nikon D3200 files. So, do a bit of research on them and you'll get a good sense as to which one you should pick!
I have a D3200, which I still occasionally use when traveling. One thing worth noting is that the D3200 originally came with Nikon's free View NX-2 software. This does not work with newer cameras, and is not as powerful as the later Capture Nx-D, which is also free, but it's reasonably convenient. If you want to edit a Raw (NEF) file and have it stay Raw, the Nikon programs are the way to go, as most other editing programs do not save their results as NEF files.
For quick viewing and resizing, I also use the freeware program Irfanview. It's well designed for this, and one of the best programs around for batch operations like resizing, but is not very good for actual image processing.
For quick and easy editing of files for the web and the like, I also like the freeware program Faststone Image Viewer. This has quite a bit of editing power, and works nicely on most NEF files, but unlike Irfanview it does not take the original JPG settings of the camera quite as well, so if your exposures are purposely a bit dark or unusual, it will try to correct them and you'll have to uncorrect them to get what you wanted.
I'm a terrible cheapskate when it comes to software, so I have not paid for anything. Other possible choices for programs if you are also cheap are the freeware Darktable, Photoscape X, and Raw Therapee. Darktable and Raw Therapee have Linux versions as well. Photoscape has a rather nice "dehaze" feature, which can be duplicated in other ways, but is convenient. I see Spencer above has recommended Darktable, and it is a good one. Some of those programs are very powerful but also a bit tricky to learn, and for the quick and dirty stuff, especially if I got it at least very close to right in the camera, I tend to default to Faststone.
I almost forgot, one feature which is hard to find, but is present in Nikon's Capture NX-D and presumbably in Studio, which includes it, is a perspective adjustment feature. Very handy, especially when traveling and unable to set up shots as well as you would like, but remember if shooting with the expectation of later perspective correction, to shoot very wide, because the correction crops an image considerably.