Though people around here will certainly try to help I do have to note Advance wars is one of the few titles with an extensive community dedicated to modding it. For my money the main site for that is http://forums.warsworldnews.com/
and though it is not quite at the level of some of the Mario games, fire emblem, pokemon or the other games with big hacker followings as far as nice GUI driven tools go you can get a lot done and as far as I am concerned they actually do things properly more often, teach people proper techniques and document stuff properly.
Enough of that though.
What you have effectively asked is "hacking -- teach me it" which is a bit outside the scope of a forum post. I have a guide to it for GBA and DS stuff ( http://www.romhacking.net/forum/index.php?topic=14708.0
) and there is also the getting started link down the left hand side of this site which goes a bit more general.
As for NARC files they are a common container format that many things can open. The big four tools are narctool (be careful as there is a pokemon specific one, usually noted as having a p in the version), narc explorer, crystaltile2 and tinke. The latter two in that list are general hacking programs but they have support built in.
By the way carc = narc file but compressed, typically with a common GBA/DS compatible compression for which there are also several tools (crystaltile2, tinke, Cue's compression tools - http://www.romhacking.net/utilities/826/
and DSdecmp http://code.google.com/p/dsdecmp/
being my usual selection).