I clearly remember walking into the local NSS newsagent and seeing the cover of Justice League of America #167 calling out at me from the bottom shelf. I later managed to pick up #168 too, a rare two-issue run for me as a kid. You could never guarantee which titles you could find and even if you could, limited funds meant you'd have to prioritise what you could find, coming back later for additional purchases when your coffers filled up a bit.
Anyway, those JLA issues (introducing me to the Reverse Flash, who I instantly loved as he was like the Flash only yellow, my favourite colour as a kid...though now it's black, so what does that say about my psychology?) were the ones that were later revisited in Identity Crisis but more importantly they were the ones that I always remembered as being my first "collected" comics.
Well, until recently that is as I've had to check the Grand Comics Database to check dates because I also recall another first comic that screamed out at me. This was Brave and the Bold #151: I distinctly recall the split screen cover, with Flash running away on the cosmic treadmill. It was the Flash that caught my eye and soon this would become my first favourite series (and Jim Aparo became my favourite artist as I was taken with the way he drew Batman's noggin for some reason. Later early favourite artists included Dick Dillin and Sal Buscema...until I discovered John Byrne's X-men in Rampage Monthly and everything changed).
With these baby steps, I had started my own comic collecting...