I’m not sure if there’s any consensus among historians about when the first Steve Jobs keynote happened–or, actually, if anyone has even asked that question until now. I like to think that it would be reasonable to say that it was in August 1976, when Jobs attended a computer fair in Atlantic City and demoed the Apple II, which was still a work in progress at the time. (Jobs wasn’t the keynote speaker at the event as far as I know, but that’s OK–he regularly gives Stevenotes these days that aren’t attached to conferences, so the definition of Stevenote encompasses any public presentation by Apple’s co-founder.)
As far as I know, there’s no surviving video from that Atlantic City show, and it’s possible that no footage exists of Jobs showing off later Apple II variants or the Apple III or the Lisa. But we have documentation of him talking about the original Macintosh at the time of its introduction, in the handy-dandy embeddable form of YouTube video.