First guitar: Little Girl, a Gibson LG0 from my mom's friend's garage sale. It was a perfect bargain: she didn't know what she was selling, and I didn't know what I was buying. Soon after, I got one of those guitar magazines that tabs out popular songs. I was in way, way over my head. But one of the songs in there was simple enough that I could muddle through. It was "What's Up" (it was the 90s, after all). For a lot of people, it's a karaoke default; for me, it's the first exposure to suspended chords. Joni Mitchell calls them "chords of inquiry...chords that [have] a question mark in them." I still like them a lot.
"Something": the first song that made me understand elegance in songcraft. (We spent some time analyzing it in Charles Kim's theory/composition class at Old Town School, which I can't recommend highly enough.) The descending line within the chord progression is gorgeous, of course, but it's also incredibly playable. For a guitarist, the song is about as intuitive as it gets, with each chord placement growing naturally out of the previous one. There are plenty of songs that use variations on this structure (Dylan uses it in "Don't Think Twice, It's Alright" and "Make You Feel My Love"), but this was the one that cracked it open for me. Without it, I'd never have written "Let Me Love You"—which is just about due for an updated recording with the full band.
I started listening to Big Star because they were important to the artists who mattered to me, and a couple of their songs turned into milestones. "Thirteen": first time fingerpicking and singing at the same time. (I was wrong on several of the chords, as I learned much later when I joined Butterbean to perform it at one of the Alex Chilton birthday shows.) "Ballad of El Goodo": knocked me over when I first heard it, even as I knew that it was beyond my abilities then. So reaching a point where I could play it comfortably counts as...sort of a long-delayed first, maybe? It unlocked other songs for me—for example, the four-note motif nested in the progression of the "Hold on, hold on" section is pretty close to the four-note motif nested in the chorus of "I Will Dare."
There are others, of course; maybe I'll write about them some Thursday. They'd make an odd, chaotic playlist. You don't get to pick your revelations.