I too had been playing harmonica for years and initially
I thought it would be a piece of cake to learn the
accordion...NOT!! The harmonica gives your "brain" the
insight on the instrument, but the "fingers" don't
seem to catch on. I did find it helps using a neck
piece w/ my harmonica and playing the accordion at
the same time. As far as a "first song", I think the
High Point Two Step and Madame Bozo are simple melodies
that one can master fairly easily without a lot of
ornaments. Good luck...I am up to 8 tunes now...
I would very, very highly recommend Larry Miller's "You can play the Cajun accordion". Comes with a CD with many old time tunes mostly by Allie Young (worth the price all by itself). He starts out with the basics, fingering singles, octaves, bass notes, tablatures for various tunes. Demo songs on CD follow instruction. A great start. My first song was "Colinda".
Beau Jocque's first release, "My Name Is Beau Jocque" was what I learned on. Very simple tunes. Guess you need a Bb accordion, though...
Other than that, it may sound silly, but "Toot Toot" is a great first song to learn. I'll bet the melody is already "something you can hum", and that is the key to learning *any* song, brutha! -- Cajun, zydeco, rock & roll...