I am new to SO. I was wondering why the initial rep is 1 (and not 0) on any new stack exchange site that I join?
You're a member, you have basic privileges (such as asking and answering questions), as such you have positive reputation.
Zero would suggest the system has zero confidence in you and that's not true; it has limited confidence in you.
Not strictly related but you only start at 1 on new sites while you don't have at least 200 rep on one site; once you get 200 you'll get an association bonus of 100 on every site you're a member of.