As for the Topic of the Day...
First Sondheim Song:  "Send in the Clowns" - It just had to be since it's one of my mother's favorites.  Of course, when I first heard it, I seriously doubt I knew who Stephen Sondheim was.
First Sondheim Show: Sweeney Todd at Toby's Dinner Theatre in Columbia, MD.  An amazing production, and her (Toby's) solution for disposing the bodies "in the round" still entertains(?) me.
First Sondheim Cast Album: Sweeney Todd
First Sondheim Show on Broadway: Into the Woods
First Sondheim Show I worked on: Company - What an amazing summer that was!  The show was being produced by Arlington (VA) County as their annual "big musical" at the Lubber Run Amphitheatre - outside, and open to the public.  Full cast, full ochestra!  (including two keyboardists and two(!) percussionists), and a sort of full set.  The director was from Australia, Brisbane I believe.  Our Joanne was brilliant.  They fired our original Amy just a few days before opening due to creative differences - and I still remember walking into rehearsal that night, and going over the "Getting Married Today" scene with the new Amy - watching the "old" Paul trying to hold back his anger and tears over what had happened...  Then on opening night, Amy1 was in the audience wearing a t-shirt with "The Original Amy" on it... Then coming to the theatre/park the next day and seeing "Parental Advisory" signs posted - apparently, they had a lot of phone calls!  And our closing performance got rained out, but we had a great party!  -Oh, and I actually got paid, so, technically, it was my first professional theatrical gig too!
First Major Professional Production of a Sondheim show that I worked on: Sunday in the Park with George at Arena Stage in DC.  It starred bk's own pals, Sal Viviano and Liz Larsen.