I don't really seem to have caught on to much starting late, but I started watching some orchestras on YouTube, while going on my laptop. You can find stuff you like or are interested in. If you really know nothing, I suppose you could either take a class or read up on music online. I wish I knew what was good.
I caught on to some things studying music in high school, too, basic things like vocabulary and things as opposed to just nothing. They were special for people who played music already, though.
One thing you could do is look up the different time periods, with debatable start and end dates:
Medieval / Middle Ages Music
Renaissance Music
Baroque Music
Classical Music
Romantic Music
Contemporary Classical Music
People don't usually seem to consider music before Baroque music often, at least in the classroom in my experience in the US. Baroque music is considered very significant, though, and is taken for normal repertoire. However, I think the orchestra might have gotten bigger in the Classical period, though I am not sure. When people say "Classical Music," they mean it as a genre and not the time period I listed.
There are many people who like classical music who don't play, I think. They seem to know a lot of repertoire.