HTML
From Wikicliki
I first got the internet in 1995, when I was in primary 5. I was really excited about how it worked then and I soon got onto Geocities, Tripod, Angelfire, Crosswinds. Making simple websites out of html which I handcoded in notepad. This was basically my start into making things for the internet.
I think it is invaluable to understand the basics of HTML and code in order to become a competent designer. There is no way to be serious about what you do without fully understanding the tool.