This is my first time installing any version of Linux on a computer and I was hoping for some help doing it as Debian's website has very little specific information on setting up Debian.

I think I want to do an install from the internet but it is for an extremely old computer which has USB ports but no Ethernet ports.

I am assuming that since it is a Pentium III (700Mhz) with a Nvidia Riva TNT2/TNT2 pro graphics card, I will need the i386 ISO image called "debian-31r1a-i386-netinst.iso".

I have DSL but I don't know if my DSL modem is supported. It's a "WireSpeed" DSL modem from "Westell." It allows the use of a USB to connect to the computer and since the computer that I'm trying to install on has USB but no Ethernet, I suppose that is really my only option.

I would like to know mainly whether I have chosen the right files to download and whether my modem will be supported for installing through a network with a USB connection.

Thank you for reading my question.

You have picked the correct iso, you could also just use the netinst floopy and base floppy.
you may, if using the floppy, need to have also the drivers floppy.

yes the westell modem is supported in linux.
and so is the Nvidia Graphics card.