http://arduino.cc/ is good place to start. It includes articles about how to get started, library references, and user forums. The Arduino IDE will certainly get you started, but as an experienced developer you'll most likely hit its limitations pretty quickly.
Later, or quite possibly sooner(!) you can use same toolset (compiler, loader, libraries) in several less limiting environments: Eclipse IDE, edit/make at the command line, and Atmel's AVR Studio. C and C++ are mainstream; I've seen references to the existence of (but never tried) AVR Forth and AVR python.
Though you'll see occasional references to an 'Arduino language' it's really nothing more than C++ with some attempt to be smart about detecting references to library packages and auto-inserting the relevant #includes.
You probably won't want to take advantage of such shortcuts, and keep to good programming practices, if you think you'll ever want to move up from the Arduino IDE.
I personally use Eclipse and(supplementing with a few command line tools) and I'd recommend it once you've tried the Arduino IDE.