Uh, dunno, there's a whole lot of it
If you're looking for other piano music, the rest of the preludes are worthwhile, you may have heard No. 4 before; the 3rd movement from Chopin's Sonata No. 2 is also probably familiar, as is Beethoven's Moonlight Sonata (Piano Sonata No. 14). Satie's Gymnopedies are also good. Saint-Saens Carnival of the Animals is probably in the right vein, lots of short, catchy pieces. I'm pretty sure everything Mozart's ever written is catchy
, if you want more melancholy stuff then his Requiem Mass is brilliant. And everyone has to know Beethoven's 9th Symphony
Beyond that, I don't know. Start with what you know, I guess. Disney's Fantasia films are a good start, and despite how tacky they are those 'Most Relaxing Classical Music EVER!!1!' sort of CDs are good for exposure - you can look further into composers you find interesting etc. if you like. A lot of classical music takes a couple of listens to get your head around, particularly symphonies and the like which are what I mainly listen to, but the most immediately gripping are also generally the most famous, so looking it up when you hear something you like just as you're doing now is probably a good modus operandi
But yeah, if you want fully to 'get into' classical music rather than just to hear well known pieces, just dive in, really
Find a composer you like and listen to stuff by them that you haven't heard. You're unlikely to find something that's genuinely bad, some might just take getting used to...
