How does one animate? I haven't a clue because I'm only beginning to learn it. I personally think it's harder than modelling, but the same could be said for anything new you try versus something you already know.
for the HL engine u just move bones around and the model will follow the movement of the bones ..
open up milkshape/max, load some model that has bones .. press the square button at the bottom-right that says "animate" .. start moving bones around .. expermint with it for a while .. not that hard