without ever hearing them before I bought Led Zeppelin IV cause I saw it on a Rolling Stones (Mexican issue, which only ranked the first ten places) list of the greatest albums ever, along with Physical Graffiti. I didn't buy PG cause it was more expensive, but my uncle bought it for me a few days(?) after. Afterwards I think I got II if I remember correctly. Not sure.. long time ago, I think I was 12 or so.
Obviously, Black Dog was the first Zep song I heard. Don't remember ever hearing them before.