Football:
The first game I watched was Norwich defeating Bayern Munich and then being knocked out by a young Dennis Bergkamp for Inter 
I had only just turned 5 and so I only have vague memories of it, but I didn’t really have much context for it as my parents weren’t really watching at the time, but I do recall there being excitement around the city for it and is basically what begun my interest in football and as a Norwich City fan. All downhill from there of course but at the time I just assumed Norwich being in Europe was a normal thing to expect (it was the tail end of our most successful period, tbf, where we’d have already qualified for Europe a couple of times had it not been for the Heysel Disaster ban, but I learnt that much later).
The first game I distinctly remember watching was the 1994 League Cup final where Villa upset United, obviously, at the time I didn’t quite appreciate that it was such a big deal (though I was already aware that United was the big bad at the time) but I vividly remember United’s Green & Gold strip looking similar to Norwich’s and Villa’s Muller shirt.
I then watched pretty much every single moment of USA '94 that I was able/allowed to, had tapes of the stuff so became pretty obsessed with it, remember cheering on Ireland beating Italy in the opener even though Roberto Baggio (yes, 5/6 yr old Sev had rat tail for a while as a result) and a young Henrik Larsson immediately became my guys though I also enjoyed lads like Gabriel Batistuta (and being confused as to why his strike partner had suddenly been sent home from the competition), Finidi George (and that Nigeria strip) and Hristo Stoichkov.
I also remember getting in NBA after MJ returned from his year off/Space Jam and being kinda amazed by that Bulls team’s dominance (I elected to be a Seattle SuperSonics fan though I remember picking up a Charlotte Hornets shirt from somewhere so also liked them).
I have some vague memories of NFL in the late 90s/early 2000s too, partly due to playing Madden and remember John Elway riding off into the sunset, but I didn’t really get into it until the “helmet catch” Superbowl in 2008
Somewhat morbidly my earliest memory of Wrestling is Owen Hart dying, and vaguely The Undertaker Dark Wedding/Ministry of Darkness/Corporation/It’s Me Austin! Bollix, but I distinctly remember the whole who ran over Stone Cold? and Triple H stealing Steph from MY BOY TEST so I guess we can firmly ascertain that November 1999 was when I started watching regularly 