I actually have no idea when I first watched it, I've been a fan as long as I can remember. I'm pretty sure the very first time was when I watched an old, battered tape one of my Auntie's boyfriends (she goes through them like I go through SW books) brought one day.
I was probably (remembers that this was before school) four or five then. Of course, I've converted to the Dark Side since then.
Twas A New Hope, no sense in starting with any of the others, eh? This was a'course before the prequels.