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.

Setup

Here is a good guide on set up and install of these tools:

http://wiki.devkitpro.org/index.php/Getting_Started

Tutorials

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

Help

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://forum.gbadev.org/

http://forums.devkitpro.org (the libnds section is the one you will want to look at)

Reddit