Debian is good, but has a steep learning curve. It's worth it though.
The current Slackware will run well on that thing as well and I think it is a lot easier to learn. I have it running quite well on a 486DX-100 with 32 Megs of RAM.
Personally I don't like old software, unless you really have to run it. If you are trying to learn, you might as well learn the newer stuff anyway
Hopefully that 486 has a fair amount of memory in it. 8-16 megs will run the non graphical "text" mode well, but forget X11 unless you have at least 32 megs, then still forget the "fancy wm's like KDE and Gnome, use Blackbox, WindowMaker, IceWm, or Xfce instead. Also forget Mozilla and use Opera, dillo, or browsex instead.