I learned how to design at first because I used to play AoK. I wasn't able to get online, so I just started making my own maps to amuse myself in single-player campaigns. Eventually I learned how to use triggers, and eventually picked up a scenario designing booklet.
Then AoM came out. First thought in my mind: That editor better be good... Then I plodded my way through the campaign and started getting decent at sup, looking at balancing junk. On January 10th of this year, I officially got a broadband connection. Eagerly I got on, shot up to a 1630 rating, then got distracted by all the cool scenario stuff. Proceeded to start making maps again. Only actual productions that have been placed on ESO are my two Escapes, the first being relatively simple, and the second incorporating several higher-level elements. And as for a third thing to release on ESO, well, that will be in awhile, gotta finish designing it first
Back on topic: Start fiddling around in the editor and figure out what certain things do. Make sure to become aquainted with what most trigger conditions and effects do, and also start finding tutorials on how to eyecandy.
As for a map type to practice with, escapes are simplistic enough and relatively easy to finish (but balancing them is far more difficult, imo).
Something something a quarter of a century old.