As for me, I can't be sure which was the very first, but for the sake of argument I'm going to say that my very first 45 was the song "Mandy" by Barry Manilow. Yup. I admit it. I was in 6th grade and it was 1975. (I'll save you the pain of doing the math...I'm just about 47 years old). I actually purchased a number of 45s around that time including another Manilow song, "It's A Miracle," as well as The Doobie Brothers - "Black Water" and America - "Tin Man."
As for Manilow, what can I say? This was early in his career when he was making the transition from successful jingle song writer to pop star, and before he went the lounge singer route. I was only 13 at the time and I was just getting into music and taking my cues from Philly top 40 music stations like WIFI 92 and WFIL/WIP. Plus, there was a cute girl in my 6th grade class at Jarrettown Elementary School by the name of Mandy (couldn't remember her last name if my life depended on it...). I'm sure she doesn't remember me! Anyway, that's my song!
Here's a video of the song from Midnight Special, a great rock program from that time period.
Now it's your turn! Step into the Way Back Machine and tell us the first song/album you can remember buying/receiving when you were younger.