i really liked pokemon as a kid. so a lot of the earliest stuff i picked up was related to that? that was like 20 years ago though so i can't remember individual words. i'd started teaching myself the language by age 12ish, so at this point i can't clearly remember a time before i knew any japanese. it's just fundamentally part of my life :)
For me, it was probably へのへのもへじ, if that counts! There was a character in a show I liked whose face was a doodled-on mask, and occasionally he would make that face. I thought it was a cute doodle with a funny name, so it always stuck with me.
I learnt 'otosan' and 'okasan' before studying when I was younger because I was watching a show that was japanese and I heard cat in that show call it's owners 'otosan' and 'okasan' a lot.
I started to watch anime at 4-6 years old, so I learned words like 'arigato' and 'ohayou', then when I was 8-9 years old I started to listen japanese music and I learned 'ichi, ni, san' and 'seeno!'...
さよなら was the first word I knew from the original 'Sing' movie where Mike (the white mouse) said 'sayounara' and possibly a bad word at some gamblers while escaping or something. (I havent watched it in a long time) And back then, I thought sayounara was a bad word until I started learning Japanese (Spare me, i was only 4 then)