The first manga I bought was volume 31 of Berserk, the scanlations I was using had a huge gap that started at that chapter so I said wtf why not.

My first full series purchase was the full box set of A Silent Voice, top 10 all time for me so I had to have it, sadly got ruined when my old home flooded but hey what can you do lol

E: Actually if you count issues of Shonen Jump as physicals then I'd been sporadically reading chapters off those when I was younger, they used to sell them in the local blockbuster I believe, though it was never consecutive so I couldn't actually follow any stories lmao. I vividly remember reading chapter #504 of Naruto and being very, VERY confused lmao