I'm friends with some super big bowie fanatics. Personally, I've always liked some of his stuff, but I've never been anywhere near as fanatic as either of them, haha. But anyways, most of his albums are super different from one another. I'm taking his discography slowly, personally, as I don't want to overload. It looks like you have the same idea. Ziggy Stardust is an easy place to start, as it's pretty 'normal' compared to lots of his other stuff. I'd consider it a pretty classic 'rock' album, to use a super general term, and I know quite a few people agree with me. Some pretty rad songs on it.
Hunky Dory and Aladdin Sane are considered some of his greatest works, from what I've heard. I've never actually listened to them yet, but sputnikmusic generally votes the same way I do, so eh. The only other albums I've listened to are Young Americans (which I originally found hilarious to listen to, as it sort of hits an old funk chord, but now thoroughly enjoy) and Scary Monsters (which there isn't a lot to say about. It's good, but I wouldn't start with it).
That's about all my knowledge about Bowie, really. A bigger fan could point you in a clearer direction, I suppose, but I figure just start with Ziggy Stardust and you'll be fine.