devkitarm (part of devkitpro) Go download it! (libnds should come along, you need it bad!)
some program to write c code in... (if you plan on doing it in c, like me) You can use notepad, or whatever flum...
and the command window... (cmd).
Not much more is needed actually!
Much info and help and tutorials at drunkencoders.com and gbadev.org
(the gbadev forum that is)
best of luck, and any more questions, go ahead
EDIT:
Okay, some basic info.. do you know c ? it helps quite a lot, but isn't that necessary if you know any other programming language since if you write your code in pseudo code, you can easily translate it all into which ever language you want, by looking it up in a c book for example. (you don't even have to know ANY language, but it will make it a bit harder.. not impossible though! But if you know some language, you will have some structure help probably..)
CODE... and then it should loop eight times, calculating 4 + x where x increases from 1 to 8...
TRANSLATE!
...
for(x=0; x