The GBADev forums would be a good place to start, and the GBAtek page has all the documentation you should need on the DS hardware itself. Make sure you have a decent grasp of C++ to begin with before working on the DS.
If you're willing to spend a bit of extra time, check out the TONC tutorials for GBA programming, as they're considered some of the best. Most of the knowledge gained there can be used on the DS as well with little to no modification.
Of course, take my advice with a grain of salt as I've never actually programmed anything for the GBA or DS with C++. I've used Dragon BASIC for the GBA a few times, but that's about it.
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