In addition to this wonderful resource, I can also recommend getting Dexed and just playing around. It's free, open source and, importantly, as unintuitive as an actual DX7. Great for digging deep into how and why a change in an operator creates a change in a sound.
As I noted in another comment, you can start out, and get pretty far, with just a single operator pair, which basically determines the waveform and spectral complexity. For me it has a similar feel (unsurprisingly enough) to analog FM, ring modulation, or oscillator sync.
