Firstly, Ubuntu is not Debian.
It is merely based upon the Debian architecture, and there's a world of difference between the two.
If you want to go Shuttleworth-free (which is my personal preference), you will find some good information on installing Debian 9.1 in a dual-boot configuration with w10, you will find some helpful information over here:
http://forums.debian.net/viewforum.php?f=17 and this thread:
http://forums.debian.net/viewtopic.php?f=17&t=134193 might be the perfect place for you to start.
I, too, have an HP desktop computer that a neighbor was throwing away, and was able to rescue it from the Wednesday morning trash collection. (It works just fine, but wouldn't run a particular game he wanted to play, so he tossed it and got something better.) Unfortunately, of course, it has windows on it....8.1 I believe.....and I'll be doing the exact thing you want to do with yours, and will be posting to the above mentioned forum regarding the problems I run into in trying to dual-boot the system--just to do it for the experience of doing it.
Others have done successful Debian/w10 installations, and have detailed their experiences there.