For general purpose GBA hacking in English it is pretty much here and GBAtemp. Some of the forums like emutalk, and things like maxconsole and dcemu and the like will be able to point you in the right direction but if you want to find the bigger groupings of GBA hackers then it is those two. That said https://jul.rustedlogic.net/forum.php?id=37
(two boards that have a fair bit of intertwining history and endless drama but you can go look up that for yourself). Some of the cheat making sites have some people which know their way around the GBA, indeed I would suggest all GBA hackers have http://doc.kodewerx.org/
(in case some forum searcher finds it in years to come and it is down then search for enhacklopedia) in their bookmarks.
If you are wanting another language then any of the regional equivalents to GBAtemp will usually have something (gbarl.it for instance for Italian, https://board.romresources.net/board219-rom-hacking-tools-tutorials-ressourcen/
does OK for German, most of the Dutch and Belgian stuff seems to have gone, Spanish is somewhat dominated by http://www.elotrolado.net/
but there were a lot of standalone hackers with a blog, I am not sure what does Brazil Portuguese right now but http://romhackers.org/modules/news/
looks like the new home of http://www.romhacking.trd.br/index.php?board=53.0
, not sure what French sites there are now, for French you might find something on http://www.playeradvance.org/forum/
but it is a shadow of its former self and logic sunrise was great for homebrew but not so good for hacking and http://gueux-forum.net/
never had much but I will link it anyway).
Some of the older GBA related sites like pocketheaven have some things but that is more historical than not.
Chinese is probably the biggest difference here with several very large hacking sites and not much crossover with the English speaking world. Several host ROMs though so no links here.
Never found a Japanese site worth linking up like I have for others so far. There are a few Japanese tools -- several Japanese devs took over and made forks of music hacking tools, and there are some great Japanese hackers out there ( https://github.com/loveemu/loveemu-lab
For sound then you might find some stuff on https://www.hcs64.com/mboard/forum.php
. It is mainly sound though, and to quite high level at times (I am no stranger to digital signal processing and electronic audio but they are frequently playing a level or two above me, and possibly several levels above what my ears can detect).
Some of the sprite making forums know enough to be dangerous, nothing I especially want to link as part of this exercise but still http://www.vg-resource.com/
and the network covers might see you learn something if you want.
For pokemon then http://www.pokecommunity.com/forumdisplay.php?f=289
along with the stuff in bulbapedia. There are a few others but most are not a patch on those two. Most pokemon hacking revolves around using tools to mod the game rather than what most of us around here would call hacking, at the same time though I have seen many tutorials for pokemon hacking start down the path of assembly hacking quite early on. You kind of have to for pokemon as it is all nicely obfuscated and the more simplistic tricks that work on most games fall short there a lot but hey.
For advance wars then http://forums.warsworldnews.com/viewforum.php?f=32
Fire emblem has a few things. Always drama there but http://serenesforest.net/forums/index.php?showforum=8
do well as jumping off points.
Not so many for GBA Mario specifically, mainly as they were mostly ports. Later mario games, including mario kart, possibly Mario Party, often have their own wikis ( http://wiki.tockdom.com/wiki/Main_Page
) and forums ( http://nsmbhd.net/
) but not really the GBA ones. You might find something at sites like smwcentral though. Similar story for Sonic (find some Sonic hack sites and you might get something, the sonicretro site came up in a search I did earlier for sound related stuff so there might be something.
Equally the phoenix wright games started on the GBA (called gyakuten saiban) and though they rose up on the DS and got hacked on those there are still those there looking at the GBA entries.
Fire pro wrestling had a decent community when I looked a few years ago. It was small but dedicated and had some decent hackers. Not the forum I was thinking of but http://tigermasksblog.blogspot.co.uk/
is not a bad start.
Mother (3) has some good hackers as it was probably one of the harder GBA games to translate. I don't see so many people hacking and modding it for stats and levels though.
Not so game specific you also have http://www.insanedifficulty.com/board/index.php?app=forums
. Though I would probably not go there to find people to learn from at the same I am not going to diss any of the hackers there.
Final Fantasy has some nice hacks, as does some stuff in the Dragon Quest family, but most of that is a few individual hackers unlike some of the other Square (Enix) games you see elsewhere.
Megaman kind of has some stuff, though more on the battle network front than the various platformers from what I have seen. Not sure what I want to link for it though. http://www.therockmanexezone.com/
is offline right now but might get you somewhere when it comes back up.
A lot of those sites do not cross pollinate each other so I have fished out quite advanced tools and techniques from them at times, and other times found them sticking with techniques there were rendered obsolete 3 years prior as far as I was aware.
With all that said a lot of GBA hacking that you might see is individuals with a copy of http://problemkaputt.de/gbatek.htm
, no$gba debug or VBA-SDL-h (no$gba is free now so probably best to go with that), a tile editor (I like crystaltile2 and http://home.arcor.de/minako.aino/TilEd2002/
), some compression tools... all sorts of things I linked up in the docs I linked you before ( http://www.romhacking.net/forum/index.php/topic,14708.0.html
"I don`t have teaching material. I have bit question to hack....
I have to belong to Mechanics school?"
I don't quite follow. You don't have to be a school trained programmer or anything to do hacking. Plenty of amazing hackers here and elsewhere are largely self taught -- I am not computer trained in any real form (very basic matlab which almost nobody ever used after the class it was taught in and microsoft office training for complete idiots is about as far as I have been formally taught). Indeed some might even argue against being school taught or that schools would not teach you much of worth if you wanted to get into hacking.