Hi James,
There is a lot to discover! I am not an expert, but here are a few tips that worked for me:
I think it pays off to read just a little bit more about the main periods in classical music, with their composers, and then listen to, say, orchestral works from these periods. It allows you to find out what you like, and also put some order in your mind. Start with Baroque (like Vivaldi, other Italians, Bach - orchestral suites or Brandenburg conc's for instance, Haendel, Telemann), Classical (Mozart Haydn symphonies), early Romantic (Beethoven, Schubert) and late Romantic (Brahms, Dvorak) up to the massive works of Mahler and Bruckner. You will soon discover what you like and do not like (and things you do not like now may come back in the future). You can do the same for solo concerto's (piano, violin), chamber music, vocal music, church/religious music, opera.. but take your time.
Concerning performances there is quite a difference between, for instance, a Mozart played by a fairly big orchestra, and the same work by a small ensemble, often playing on instruments of that time. I just listened to Vivaldi's 4 seasons, by Janine Jansen - one of >200 available recordings apparently. Have a look at amazon reviews to read how different opinions can be.
Finally, a few of my (old) favourites you might perhaps also like (I admit these are not cheerful works though..): Mozart's Requiem, Brahms Deutsches Requiem, Opera: Purcell's Dido & Aeneas, Gluck's Orphee & Euridice - (some great dvd's around also).
Enjoy!