Aye, you've asked a pretty general question, and the answers are spot-on.
You can do a lot of customization in SQL, pretty much all items and mobs. If you want to change the core mechanics of the game, not content (see above), then you're probably going to start needing to dabble in C++. Fortunately, the code's pretty well-documented (IMHO), and we all have to jump in some time. But before you do any of the source mods, consider getting your feet wet with items, mobs, and zones; i.e, the content before the laws of the universe.
And all of this should come after you check out the wiki's decent documentation of the rules and variables options. They're separate tables where the variables system is gradually being replaced by the rules architecture, but both seem to sort of apply, so you should probably get cool with knowing what both can do.
When you can master the variables/rules, the mobs, spawning, and items, then you're ready for source code b/c you'll have no other choice to do what you want to get done. Before that point, though, you're probably putting the cart before the horse.
I say all of this b/c I've only just know begun to realize what I can't do after 3 months of enthusiastic modding. I'm still pretty much ok customizing the DB and pretending I was Angelox or CaveDude from about 3 years ago.
Anyway, good luck!
D.
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