I can't remember what my very first comic was. I can narrow it down to one of three, all cover dated July 1978, when I was 4 years old!:

SUPERMAN # 325(first of a two-parter with Blackrock-cover shows Superman crushed by a gigantic hand).

ACTION # 485 (reprint of the early 70s story where Superman takes a bite of Kryptonite-featuring the now-famous cover by Neal Adams, with Superman snapping chains against an orange background)

DC COMICS PRESENTS # 1(part 1 of another 'race' between Superman and Flash)

This was in the era of 'Whitman' editions in plastic packs at drug stores, so for all I know, all three of these were in the same bag! But then, it was also common for consecutive issues of the same title to be packaged together, as well.( I seem to remember the next two issues of SUPERMAN were together in another bag).

This is the first comic I can remember, and I don't even own it :-p I don't own any of these early X-Factor issues. I would like to eventually find this comic again so I can read it and see if its any good .

Wow.  Bradley just made me feel old.  His first comic has a Liefeld cover and he referred to issue 50 as an "early" X-Factor!  Sheesh!

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hahah whoops.

I started collecting in the 90's, and this is how I eventually found Byrne, because so many comics in the 90's were crap. I was being drawn to Byrne comics over and over again without relizing it. Finally I caught on to his drawing style and was surprised to find I was unknowingly collecting his runs of different characters.

I'm one of those who can only narrow it down. :)  Based on vague memories of picking these up on the newsstands and looking at checklists, it's most likely one of the following:

a) Marvel Team-Up #106 (Spider-Man and Captain America vs. the Scorpion)
b) Marvel Super-Action #32 (reprint of Avengers #71 - Full line-up of Avengers vs. Kang and the Grandmaster; guest starring Marvel's Golden Age Big Three)
c) Superman Family #207 (which had the Legion and Earth-2 in it so I believe conventional wisdom says I should have run away screaming from DC and never looked back :-) )

I also would have gotten Roger Stern's Spider-Man book about this time, so it might even have been that if you want to count it.  (Reprints of AF #15 and ASM #80.)

(Edited to add images.)