Sooo Im just gonna dump some info from my nightcore making days that may be of use to someone
Firstly nightcore can range anywhere from 20-30% speedup depending on the song and what you're going for. Beyond that its fairy open, some nightcorers would remix songs a bit, but most of them just did a seedup.The program you use to do the speedup will have a big impact on the style of your core.
Audacity is the classic - I don't know how much their algorithms have changed, but you could always download an old 2010 version to be 100% matching the classic nc sound.
There are two ways to speed up a song in Audacity, you import the song, select it, then you can choose
"Effect/Pitch & Temo/Change Speed and Pitch..." - you can then select a % change so enter 25%. HOWEVER, some people recommended
"Effect/Pitch & Temo/Sliding Stretch...", this effect lets you have different pitches and speeds at the start and end of the song, however you can just enter 25% in all four boxes.
Audio differences are always subjective, but I do think there is a tiiinnyyy difference, the Sliding version sounds softer to me. I made some samples!
NC of I Love It made using "Change Speed and Pitch"
NC of I Love It made using "Sliding Stretch"
I also recommend exporting your nightcore as a WAV or AIFF if you plan to import it to a video program to add a picture; its gonna be compressed again by the video program so there's no need to compress it twice unless you want it to be crunchy.
As for the video - any video program will do; I think most small nightcore people were prob using iMove or Windows Movie Maker, while most "professional" channels were using Adobe Premier - the channels that had fancy animated audio spectrum graphics on their videos did that with Adobe After Effects (You can get both in the CS6 Master suit that can be found around the internet) - the basic audio spectrum is built into AE, but you needed extra plugins like Trapcode Sound Keys for bass response animations. Some video programs will also let you change the sound speed in the video program (I use Final Cut these days and it does a much cleaner job of changing song speed than Audacity, but I find it too clean, it does not sound like NC anymore
)I'm of the opinion that a nightcore only becomes a nightcore when its in a video with an anime pic and on youtube, but its ok if you just like the audio part
As for songs; the general advice is that A cappella, folk singer, electric dance, rave/trance all work quite well - house music does not work; generally deep bass songs don't work well and songs that have a high range of speeds (where some parts are fast and others slow) don't work well. You want a song with a consistent steady speed, clear-already high vocals, and a solid tune!
Finallyyy I should mention the Nightcore Forum that is well worth reading up on! https://www.nightcoreuniverse.net/viewforum.php?f=17 (if you can get it to load)