Well, my first map was probably for Heroes 2. I can remember that I have spent more time in the Editor than actually in the game itself. The terrain painting and the overall cuteness of the game was just too addictive That was in the late 90's, then I just remember the fragments of messing with Starcraft editor and Aurora toolset Beta (Neverwinter Nights), but never released anything, until 2004. Then it became pretty serious with Hammer meeting the Natural Selection and since then it was pretty solid addiction until now
It took me over a year to pull up my first map for NS1. I have made all the classical beginner mistakes, same mentioned in this topic , and I have had to scrape all my work several times due to the technical inexperience with Goldsrc. In the end, I have decided that I should to focus on smaller project first and gain some experience before moving onto something more ambitious. So as my first map I consider co_Overmind - combat map for NS1, but that was just a springboard to produce ns_Hulk, for which I was aiming (unsuccessfully) from the beginning.