I'd start with playing with flash between the shooting modes, and seeing how flash behaves differently when switching the camera between P, A/S, and M. While you may have mastered the exposure triangle for ambient-only lighting, adding flash is a whole new ballgame, and something as basic and simple as switching your camera's shooting mode can have a drastic WTF?! impact. A and S assume you want fill flash (mostly ambient, little bit of flash), so unlike P&S flash cameras, you can still have very long shutter speeds. P takes the ambient light level into account and switches to using the flash as the main source of illumination in lower light levels. While M lets you do whatever you're gonna do.
Starting out with the camera in M, and the flash in M will give you more visibility into what's actually going on than being in, say, A and iTTL will, and may be faster for learning.
You'll also want to learn about FP flash (if your camera body supports it), and certainly about my max. sync speed if your body doesn't have FP flash capability.
I'd start with on-camera bouncing, with and without the black foamie thing.
Off-camera flash is nifty keen, but start small and simple with the least amount of equipment to me is the easiest way to learn the basics. Exhaust what you can do with a flash on-camera before you take it off-camera. Learn about tilt and swivel and what moving the flash head to change the angle of light can do; learn what adjusting the power level can do; learn what bouncing does vs. hard direct flash. Quality, direction, and quantity of light are your main controls to how the flash will appear in the final image.
Learn that while your ambient is controlled by iso, aperture, and shutter speed; your flash is controlled by iso, aperture, flash-to-subject distance, and flash power.
Try adjusting your shutter speed between shots to see what effect that has on the image. Learn how to drag the shutter.
Then mess about with iTTL and FEC, vs. Manual on the flash, and learn when you'd want to use one over the other. iTTL has advantages as well as drawbacks, and despite what those who've drunk the Strobist Kool-Aid would tell you, it's as useful a tool as having A and S modes on your camera body. You'll want to use i-TTL for run'n'gun event situations.