I got the Steinberg USB interface. With all the help you guys have given me I thought I should give you an update.
The good news is that it is working! I can hear YouTube and my guitar at the same time. It even works with Amplitude, so I can have custom sounds while doing lessons.
A small problem is that, because of the drivers, I can't plug the headphones into the computer, only the jack on the interface. That means I can't use my favorite wireless headphones, at least for now. Maybe I'll be able to figure out a solution for that in the future.
Still, on the plus side, the sound is excellent and having the mix knob instead of a direct input switch is cool.
So, now that that's done it's time to get cracking on those lessons. :-)