Pretty self-explanatory. What was the first manga series you started reading? In my case, it was Inuyasha during my early years in college.
Honestly the first Manga I read was probably Naruto, back in the day when I was obsessed with it and it was still pretty decent. First manga I completed was either Suzuka or B-Shock, I can't quite remember the order in which I finished those two, really enjoyed both of them though.