First you will need to install devkitARM. This is a set of tools produced by devkitPro which allow you to compile your source code into Nintendo DS compatible binary.
Here is a good guide on set up and install of these tools:
Once you have your tool chain installed you will need to learn a bit about the hardware. The state of tutorials is a bit lacking but it should be enough to get you started.
Drunkencoder Tutorials: http://www.dev-scene.com/NDS/Tutorials
Patater’s Manual: http://patater.com/manual
We have a pretty active irc chatroom at irc.blitzed.org in the #dsdev channel
There are also a couple of active forums you can post on and search:
http://forums.devkitpro.org (the libnds section is the one you will want to look at)