http://devrs.com/gba/
http://www.gbadev.org/
http://www.cfxweb.net/gba.php
go here, you download a program free, you make the game on your computer with the program. then you buy a flashkit will means like you "copy" the game from your comp to a gba cartridge. this cost like 200$ (the flaskit) you can get it at
www.myroms.com or amazon.com or ebay.com i think. then you hive your cartirdge to all the publishers in the world.